(Adds cabinet being sworn in, background)
By Paul Hoskins and Jonathan Saul
DUBLIN, June 14 (Reuters) - Prime Minister Bertie Ahern cemented his position as one of Ireland's most successful politicians on Thursday by winning a third term in office and ushering the Green Party into government for the first time.
Ahern said having Greens in his administration reflected not only his desire for a solid parliamentary majority but also his concern over the environment.
"These are issues all over the world and people are grappling with climate change," he said after receiving his seal of office from President Mary McAleese.
Speaking over the sirens of his police escort, he told national radio that having environmentalists at the cabinet table would give his administration "newness and freshness".
Ahern, in power for a decade, earlier won 89 votes in the 166-seat Dail (lower house of parliament) after six Greens, two members of the pro-business Progressive Democrats and four independent deputies joined his centrist Fianna Fail in backing him after a May 24 election.
Enda Kenny, leader of the main Fine Gael opposition party, congratulated Ahern for his persistence and "permanence" and described his electoral record as almost unprecedented.
"I suppose we should all be glad for small mercies that you have decided to start the long glide to retirement," he said.
Ahern, 55, has said this will be his last term in office. He now has a political record second only to Eamon de Valera, the American-born maths teacher and revolutionary who fought British rule and wrote Ireland's 1937 constitution.
Finance Minister Brian Cowen, widely tipped as Ahern's successor, was one of many ministers re-appointed by President McAleese and he assumed additional duties as deputy prime minister.
Two Greens were among the new cabinet members and will oversee energy and environment policy.
Praised for working with British premier Tony Blair to broker a peace deal in religiously divided Northern Ireland, Ahern spoke of a "glad departure" from past violence.
"Through every single day of the past decade, the quest for peace has been the single dominating purpose of my public life," he told lawmakers.
"Today, I pledge again as I have before, that the cause of peace will be the cause that is always closest to my heart."
Despite attacking Ahern's environmental and social record during the election campaign, Green lawmakers pledged not to disrupt government whenever there was a disagreement or crisis in cabinet.
"There will be a huge amount of tension throughout the five years," Green deputy Ciaran Cuffe said before Ahern's selection.
"Have a clear list of deliverables and don't walk out the moment tensions rise," he said of advice received from other Greens who have served in governments in Germany and Finland.
By drawing on support from a number of quarters Ahern has diluted the influence of any one partner. While the new government will have a greener hue, no one grouping in the administration will have enough power to force his hand.
Ahern has been prime minister or finance minister for 13 of the last 15 years during which a once impoverished Ireland became one of Europe's wealthiest nations. But such rapid economic growth has contributed to environmental problems.