[7] He served as Green Party Spokesperson for Transport, Enterprise, Trade and Employment and Communications, Marine and Natural Resources during the 29th Dáil.

Ryan was appointed to the cabinet by Taoiseach Bertie Ahern, as Minister for Communications, Energy and Natural Resources when the 30th Dáil sat the following day.

As Minister, he stated his priorities would be to improve upon the changes underway in the Irish energy sector, with strong emphasis on aiding the adaptation of renewables and building retrofitting.

During his period in office, installed wind capacity in Ireland doubled, and by 2010 the average daily energy derived from renewable sources (as a percentage of total demand) had increased to 17%, peaking at 42%.

[10][11][12] He also committed Ireland to the European Super Grid programme in 2009 and announced major government investment in marine energy research projects.

[17][18] In response to the Corrib gas controversy, the environmental activist organisation Shell to Sea and its spokesperson Maura Harrington, criticised Ryan for joining Fianna Fáil in coalition as the terms of the programme for government did not include a reversal or renegotiation of the proposed gas pipeline and refinery at Broadhaven Bay, County Mayo.

[21] Ryan was also criticised by Shell to Sea for failing to launch an independent review of the decision, as stipulated by the Green Party in a motion passed at their annual convention in 2007.

[22][23] Ryan resigned as Minister for Communications, Energy and Natural Resources on 23 January 2011, when the Green Party withdrew from government.

[24] The following day he helped broker an agreement with Fine Gael and the Labour Party which ensured the passing of the 2011 Finance bill, prior to the holding of the general election on the 25 February 2011.

The Greens' return to the Dáil following their wipeout in 2011 was the first time in Irish election history that a political party had been able to survive losing all their TDs.

[28] Fine Gael rejected this suggestion, with Minister for Culture, Heritage and the Gaeltacht Josepha Madigan responding that her department had no plans to reintroduce wolves into the country, saying that doing so would do damage to farming.

"[35] In June 2020, he was criticised online, particularly by Green Party councillors, for his use of the phrase "you nigger" when quoting director Sean Gallen on his experiences of racism.

[38] On 27 June 2020, following 139 days of negotiations, the members of Fianna Fáil, Fine Gael and the Green Party all voted in favour of entering government together as a coalition.

[43] Ryan sought re-election, and Deputy Leader Catherine Martin also contested the position, leading to the 2020 Green Party leadership election.