(Adds details, quotes from Ahern's Fianna Fail party)

By Paul Hoskins and Jonathan Saul

DUBLIN, June 12 (Reuters) - Green Party leaders said on Tuesday they were ready to enter government in Ireland for the first time, giving Prime Minister Bertie Ahern the backing he needs to secure a third successive term.

"Last Friday, the negotiations had to conclude as insufficient progress had been made," Green Party leader Trevor Sargent told reporters in Dublin. "Now we feel the roadblocks that existed have been worked through."

The support of all six Green lawmakers, which still has to be endorsed by the party's members at a meeting on Wednesday, would guarantee Ahern a majority in the 166-seat Dail (lower house of parliament) following an inconclusive May 24 election.

Lawmakers will vote on a post-election prime minister when parliament reconvenes on Thursday.

Ahern had been scouting around for new allies after his Fianna Fail party emerged largely unscathed from the vote but his pro-business junior coalition partner, the Progressive Democrats (PDs), suffered big losses.

Fianna Fail has 78 seats versus 80 in the last parliament and could have secured a wafer-thin majority with backing from two remaining PD lawmakers and a few independents.

Additional support from the Greens will give Ahern more room for manoeuvre, however. It may also improve his chances of serving a full five-year term that he says will be his last.

The Greens had backed out of talks on Friday blaming "substantial blockages" but that changed on Monday night after a meeting between Ahern and Sargent ended with differences over climate change, education and political reform resolved.

Both sides refused to give details of the government programme agreed upon and said they had yet to decide which ministries the Greens would control.

"There was compromise of course, we have six TDs (lawmakers), Fianna Fail have 78," Sargent said, adding that people would see "considerable" achievements in the programme.

Green Party members will vote on it at a convention in Dublin due to start on Wednesday afternoon.

Ahern's social affairs minister in the outgoing government, Seamus Brennan, said the agreement was comprehensive.

"It covers a broad range of policies which the new government will get down to implementing straight away."

Finance Minister Brian Cowen declined to say if changes to his tax policies had been required.

"We are pursuing this programme on the basis of maintaining a strong, dynamic, progressive and sustainable economy," he told RTE television.

Although Greens have never been in government in Ireland, party chairman John Gormley said he believed their recent involvement in administrations in Germany and Finland showed such coalitions could realistically make a difference to policy.

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