A Sinn Fein senator who forced the Irish government into a bruising by-election was last night celebrating a landslide victory in one of Fianna Fail's strongest outposts.

Pearse Doherty (33) bulldozed his way to a seat in Donegal South West with an almost 40pc share of the vote in the party's first by-election victory since 1925.

It left Fianna Fail tending deep wounds in what was a hugely humiliating defeat in Tanaiste Mary Coughlan's home constituency.

Just over 55pc of the electorate turned out to vote, with Sinn Fein forging significantly ahead of Fine Gael and Labour as the main beneficiary of a decisive swing away from Fianna Fail.

The father-of-three from Gweedore topped the poll with almost 40pc of the first preference votes, almost outpolling the combined Fianna Fail and Fine Gael total.

Fianna Fail's Brian O Domhnail and Fine Gael's Barry O'Neill (37) polled 21pc and 19pc respectively in the three-seater constituency that has been previously dominated by the two main parties.

Mr Doherty was elected on the fourth count without reaching quota just before 7pm last night. His election reduces the Government's already wafer-thin majority to just two in the Dail.

Sinn Fein is now expected to open talks with independents in the Dail with a view to forming a technical group, which will give them full speaking rights in the Dail.

It will also give the party the numbers to table a motion of no confidence in Taoiseach Brian Cowen.

Last night they called on Labour leader Eamon Gilmore to rethink the party's alliance with Fine Gael.

Speaking at the count centre in Stranorlar, Sinn Fein president Gerry Adams said the electorate had clearly rejected the notion of a coalition between Labour and Fine Gael.

"I think the Labour leadership needs to review their situation. Clearly people want an alternative way forward. This election is an endorsement of that approach," he said.

Mr Doherty, who will vote against the Budget on December 7, said the result was an endorsement of the Sinn Fein vision and sent a clear message of rejection to the parties that presented themselves as an alternative.

"I would call on Labour now to change direction. You cannot credibly argue that you are of the left when you go into bed with a party like Fine Gael who are of the right. This is a wake-up call for Eamon Gilmore and that party," he said.

Fianna Fail, meanwhile, tried desperately to put a brave face on the crushing collapse of the party's vote from a lofty 51pc in the last General Election.

The defeat was all the more stinging given the major resources it had ploughed into the contest in the constituency, which was visited by dozens of senior party figures and three times by the Taoiseach in the past month.

Voting trends also pointed to a collapse in the Tanaiste's own vote in her own local area of Frosses and Mountcharles but, while conceding they were disappointed, she dismissed any suggestion that the party was in meltdown.

"We are not facing wipeout. We are facing a very difficult and challenging time and a very difficult and challenging election but I don't see the wipeout of Fianna Fail in Donegal South West," she said.

Social Protection Minister Eamon O Cuiv -- playing down the significance of his party's defeat -- said the Fianna Fail candidate had a difficult battle.

The former Fianna Fail TD for the constituency, Pat 'The Cope' Gallagher, said it was necessary for the party to "regroup" but described it as possibly the most difficult by-election for any Fianna Fail candidate to fight.

Fine Gael director of elections Joe McHugh put the Sinn Fein win down to the electorate rewarding Mr Doherty for his successful High Court challenge. Mr O'Neill, meanwhile, insisted he had fought a strong campaign and would be back to fight in the General Election.

Labour candidate, Frank McBrearty (41), who came a disappointing fourth, acknowledged he was "disappointed but happy".

Veteran Labour TD Joe Costello described his performance as "respectable".

"Labour has come from a low base of 3pc but we managed to grow our vote substantially. It is a 250pc increase on the last General Election," he said.