The Green Party postures as a weak, but sincere party of environmental idealists, but enabled the neo-liberal political coalition of Fianna Fail and the PDs to complete its term in office from 2007-11.
"It was ironic that the highest level of anger came in the final hour... It was a philosophical difference -- our belief is that the roads programme in its current form goes against the need to have climate change measures. If you're increasing private car use and you're not balancing it out with public transport, it sends a mixed message."
Green Party Senator Dan Boyle in 2007 concerning the “tough” negotiations with Fianna Fail. 
Green Party of Ireland
The Green Party was founded in 1981 and first entered Dáil Éireann, in 1989. It has served a full term from 2007 to 2011 as junior partner in coalition with Fianna Fáil and the now-defunct Progressive Democrat party, first under Taoiseach Bertie Ahern and later under successor Brian Cowan. In the 1989 general election the Green Party won its first seat, with another won at the 1997 general election, the party gained another seat when John Gormley won a Dáil seat.
At the 2002 general election the party made a breakthrough, getting six TDs elected to the Dáil with a 4% vote, however the Greens subsequently lost all six of their TD's in the February 2011 election, In the February 2016 election, they returned to the Dáil with two seats. In the 2020 general election the Greens gained 12 seats, becoming the fourth largest political party in Ireland. After negotiations, and ratification of an agreed Programme for Government, the Green Party agreed to enter a second coalition Government with Fianna Fail and Fine Gael. 
Green Political Power
The question is, what will the Green Party do with their new-found political power? In this regard, it might use useful to examine their previous record in Government, as junior partners in the 2007 to 2011 coalition government, a time which covered the explosion of the inflated bubble of the Celtic Tiger, the subsequent Bank Guarantee, and the austerity policies in the wake of that guarantee.
Coalition Government: 2007 - 2011
The 2007 election saw Bertie Ahern win a third successive term as Taoiseach with Fianna Fáil remaining the largest political party in Ireland. By June 2007, the Green Party entered Government for the first time as junior coalition partner, with the support of the Progressive Democrat party and various Independent's. The following May 2008, Ahern resigned after 11 years as Taoiseach over his financial affairs. Ahern was Ireland's second longest-serving Taoiseach after Fianna Fáil founder Eamon De Valera, Finance Minister Brian Cowen promptly replaced Ahern. 
Government Formation in 2007
"A deal with Fianna Fáil would be a deal with the devil... and [the Green Party would be] decimated as a Party"- Ciarán Cuffe TD before talks. 
The negotiations on coalition had began promptly after, the Green Party staged a walk out, however, after six days talks quickly resumed with a draft programme for government quickly agreed. On 13th June 2007, party members voted 86% in favour of entering coalition with Fianna Fáil. The next day, the six Green Party TDs voted for the re-election of Bertie Ahern as Taoiseach. The new party leader John Gormley was duly appointed as Minister for the Environment, Heritage with Local Government and Eamon Ryan was appointed as Minister for Communications, Energy and Natural Resources. Trevor Sargent was named Minister of State for Food and Horticulture.  In the wake of the growing economic crisis caused by the global (property induced) crash and the Irish bank bailout, the Progressive Democrats promptly disbanded themselves on November 2009 with Mary Harney nonetheless continuing on as an “independent” member of the government until 20th January 2011. The Green Party left coalition government on 23rd January 2011.
Policy Record and Implementation
Prior to its entry into government in 2007, the Green Party had been an supporter of the following major environmental (and heritage) campaigns in Ireland: 1. The Shell to Sea (Corrib Gas Pipeline) campaign, 2. The campaign to reroute the M3 motorway away from the Tara Valley. 3. The Lisbon Treaty. On all key fundamentals of their policies, the Green's instantly jettisoned their support for these campaigns upon entry into Government. The Green Party had used these major environmental and heritage protection campaigns to their electoral advantage in the 2007 election.
The Corrib Pipeline: Green Party member Eamon Ryan oversaw the Corrib gas project while he was in office. The Party had, at its last annual conference, made a “full independent review” into the project a precondition of entering government but immediately dropped this stance during the 2007 post-election negotiations with Fianna Fáil. 
Oil & Gas Licences: On 1st August 2007, despite Green's professed opposition to oil exploration in Irish waters, Minister Eamon Ryan duly announced a new licensing round for oil and gas exploration in the ‘Porcupine Basin’ to commence in the early autumn. Ryan's statement announcing the licence round was interesting in its own right:“Energy prices continue to rise. Both of these factors are leading to greater profits in the industry. Fewer prospective areas are open for exploration internationally, making Ireland’s unexplored basins more attractive. The Atlantic area is seriously under-explored. Departmental analysis of this area estimated risked reserves in the order of 10 billion barrels in the Atlantic area alone.” 
The Tara Road: The M3 Motorway running through the Tara/Skrene (or Gabhra valley) cost €1 billion to build. The motorway runs from Meath (still a largely rural county) to the rural Cavan border and is 60 kilometres in length. The motorways had generated sustained opposition from campaigners who at various periodic intervals had staged extensive protests along the route also in the courts to stop it going through the Tara Skryne (Gabhra) Valley.
The Irish Government announced the M3 Motorway project in 2005, with Fine Gael supportive and Labour critical. The Green's official position on the Tara project was that this massive motorway project should be moved away from the Tara valley. In taking this position, the Greens failed to argue for the cancellation of the M3 project, simply an alteration of the chosen route. This is an indication that the possible political impact of the road was the primary consideration, and not damage to Ireland's fragile heritage and environment. In that year, Ciarán Cuffe TD stated that the Green Party was considering taking legal action, this amounted to nothing and Green Party opposition to the Tara/Skryne Valley route evaporated upon entry into power in 2007 two years later. 
The Lisbon Treaty: At a party convention on support for the Treaty of Lisbon in 2008, the party voted 63.5% in favour of supporting the Treaty, short of the party's two-third majority requirement for policy issues. As a result, the Green Party did not have an official campaign in the first Lisbon Treaty referendum. The referendum did not pass in 2008, and following the Irish government's agreement with EU member states of additional legal guarantees and assurances, the Green Party held a special convention meeting in 2009 to decide its position on the second Lisbon referendum. Two-thirds of party members present voted to campaign for a 'Yes' in the referendum. In taking this stance, the Green's promptly abandoned the party' previous skepticism towards EU integration and 2009 was the first time that it actively campaigned in favour of a European treaty. 
To give a summary of the Green’s principle accomplishments in office:1. The Corrib Gas project was forced through Broadhaven Bay, with at times considerable force. 2. The M3 motorway project was pushed through by a State which was determined to display that it valued land rezoning and the automobile over Ireland's natural and historical legacy. 3. The Lisbon Treaty was successfully re-run a second time by the Irish Government in 2009. 4. In supporting these projects, the Greens proved that their professed environmental principles were secondary to the gain and maintainence of political office. 5. But it was in the arena of politics that the Greens displayed their determination to sustain the agreed political policy of austerity placed upon the Irish population in the fallout after the 2008 bank guarantee.
The 2011 Green Departure and the 6 Billion Austerity Budget
“For a very long time we in the Green Party have stood back in the hope that Fianna Fáil could resolve persistent doubts about their party leadership. A definitive resolution of this has not yet been possible. And our patience has reached an end.” - Green Party leader John Gormley, 23rd January 2011
The Green Party supported the 2008 bank bailout and the subsequent EC–ECB-IMF - financial support for Ireland's extended bank bailout in 2010. However, on 19th January, the Green party upset Taoiseach Brian Cowen's reshuffle plans. The Greens were annoyed at not having been consulted about this effort, and went as far as to threaten to pull out of the coalition unless Cowen set a firm date for a general election. The election was due in any case that spring, with his hand now “forced” by Gormley., Cowan's final election set date was March 11th. On 23rd January 2011, the Green Party met with Cowen following his resignation as leader of Fianna Fáil the previous day. The Green Party then announced it was breaking off the coalition and going into opposition with immediate effect. 
It should be noted that the Green party had continued relentlessly on in Government until well after the Bank Guarantee and subsequent “bail-out” by the EU/IMF had been implemented. The austerity and bank guarantee process was now institutionalised in the form of 1. “bad bank” of NAMA and 2. the Troika (EU/IMF) financial oversight structure with close Department of Finance support. By 2011, with the budget completed, and a general election imminent, the Greens departed. That this deepening and institutionalisation of austerity had the full backing of the Greens was made clear by Gormley's own statement announcing the Green's departure from the Fianna Fail/Green coalition Government:
Good afternoon ladies and gentlemen and thank you for joining us. On November 22 last we said that an election should be held early this year because of the events surrounding the IMF bailout. On that occasion we identified four key priorities, which needed to be addressed before this election could take place. These were: concluding financial arrangements with the EU/IMF; producing a four-year economic plan; passing Budget 2011; and passing the Finance Bill to give effect to that Budget. Today, three of these four objectives have been completed. We believe it is possible to complete the Finance Bill quickly before going to a general election.
– John Gormley, 23rd January 2011 
The 2011 budget was described as the most draconian budget in the history of the State, with €6 billion worth of cuts to the state budget. 
First steps toward a Cross-Party Coalition
In October 2010, and before the impending December austerity budget, then- Minister Eamon Ryan called for a grand political coalition to implement future (austerity) policies. Ryan made clear that politicians “cannot play the usual party politics, they all have to talk to each other, sit down and have rational discussion to reach common agreement.” Fine Gael, however rejected calls for a political coalition with the 2011 election imminent. In the aftermath of the COVID-19 crisis and the effective repudiation of the 2020 election result by the main Irish political parties led to the formation of an grand coalition Government between the old civil war rivals, Fianna Fail and Fine Gael., enabled by the Greens. 
The Light Shines in the Darkness
The Programme for Government Our Shared Future 2020 coalition framework document was agreed with Fianna Fail, Fine Gael and the Greens on June 15th last, it has been described as the most "most fiscally conservative arrangements in a generation," by a clear and distinct minority of Green Party opponents of the agreement. The Green Party in general, however, voted the agreement through with a clear majority in favour of a three - power Grand coalition Government. 
The Green Party's doleful history is of a political grouping which postures as a weak, but sincere party of environmental idealists, but which enabled the neo-liberal political coalition of Fianna Fail and the Progressive Democrats to complete its term in office from 2007-11. When the PD's departed in 2009, the Green Party promptly replaced them as a central actor in the enforcement of austerity to pay for property speculation with one of the most severe budgets ever enacted. The Green party has enabled both environmental and historical decimation (the M3 Motorway project) and the transfer of national resources to a multinational corporation (the Corrib Pipeline).
The circle has now been completed from 2007 to 2020: with two of the actors responsible for the agreement and implementation of the bank guarantee and subsequent austerity measures., Fianna Fail and the Green Party present and correct. Fine Gael can now play the intellectual role played by the previous absent partner, the Progressive Democrats, with their core policy-framework substantially intact. 
 https://en.wikipedia.org/wiki/Green_Party_(Ireland)#cite_note-15; https://www.independent.ie/irish-news/green-senator-saw-red-during-tough-negotiations-with-fianna-fail-26313164.html; https://www.rte.ie/news/2007/0603/89652-election/; https://www.rte.ie/news/2007/0610/89876-election/; https://www.reuters.com/article/idUSL17110745; https://www.reuters.com/article/us-ireland-election-greens-idUSDUB00032720070612
 https://web.archive.org/web/20110222003750/; http://www.greenparty.ie/en/news/latest_news/greens_back_lisbon_yes; https://www.rte.ie/news/2009/0718/119706-greenparty/; https://www.rte.ie/news/2008/0119/98459-greens/ http://news.bbc.co.uk/2/hi/europe/6901353.stm
 https://en.wikipedia.org/wiki/Green_Party_(Ireland)#cite_note-15; https://www.rte.ie/news/2011/0227/298128-election/; https://www.bbc.com/news/av/world-europe-12262796/green-party-quits-irish-coalition-government
 https://www.rte.ie/news/politics/2020/0615/1147519-government-formation/; https://www.bbc.com/news/uk-northern-ireland-53123592; https://www.bbc.com/news/world-europe-53045282; https://www.rte.ie/news/politics/2020/0628/1150117-politics/; https://www.rte.ie/news/politics/2020/0625/1149711-programme-for-government/