The grip on power of the Irish Republic's coalition government looks set to slip further as a by-election gets under way in County Donegal.
Fianna Fail, along with its allies, can muster a majority of only three seats, just months, or possibly weeks, before an expected general election.
Its candidate in Donegal faces a tough challenge from Sinn Fein.
The party hopes to pass an austerity budget next month, having agreed to a massive international bail-out.
"I am quite satisfied from my discussions both with the parties in government and the various public representatives in Dail Eireann [parliament] that there is a majority for this budget and that it will pass," Finance Minister Brian Lenihan said on Thursday.
Running a budget deficit equivalent to 32% of GDP this year, the eurozone state is expected to borrow about 85bn euros (£72bn, $114bn) from the EU and IMF.
Uncertainty over Ireland's ability to stabilise its finances continues to hurt the euro.
The currency fell by almost half a cent to $1.3325 on Thursday, and has now fallen by more than three cents this week.
Polls opened in Donegal South West, a sprawling coastal constituency in the north-west, at 0700 (0600 GMT) and are due to close at 2200.
Traditionally a safe Fianna Fail constituency, the seat became vacant 18 months ago when the sitting TD (member of the Dail), Pat "The Cope" Gallagher, was elected to the European Parliament.
Brian O Domhnaill is hoping to retain the seat for the party against four challengers, of whom Sinn Fein's Pearse Doherty is seen as particularly strong.
Mr Doherty had taken court action against the government to force the by-election, successfully arguing that it was unconstitutional to keep the seat vacant for so long.
The other candidates in the by-election are Barry O'Neill for Fine Gael, Frank McBrearty for the Labour Party, and former Sinn Fein councillor Thomas Pringle, who is running as an independent.
Hit by cuts
Donegal South West is a largely rural area hit by high unemployment, emigration and an ailing fishing industry.
On Wednesday, the Irish Taoiseach (Prime Minister), Brian Cowen, unveiled a drastic four-year recovery plan, promising 3bn euros in social welfare cuts, a rise in tax and the axing of 25,000 public sector jobs.
Sinn Fein described it as a plan for national recession.
One Donegal South West voter told the BBC News website he was supporting the Fianna Fail candidate only because he admired him personally for his work with community groups.
Joe Brennan, a qualified seconday school teacher unable to find employment in his own sector and now working as a web developer, said the government's cuts would have "serious consequences" for both himself and his family.
"Politically nothing will come from this by-election," he predicted.
Yielding to pressure from his own allies, Mr Cowen has promised an early general election but insists it should not take place until after the Dail votes on his budget, on 7 December.
The Green Party, the junior partner in the coalition, has called for the poll to be held in January.
A recent article in the Irish Times newspaper suggests the government's hold on power is precarious, with the support of several nominally allied TDs not guaranteed.
Including Donegal South West, four of the 166 seats in the Dail are currently vacant, meaning the coalition needs at least 82 to pass legislation.
The coalition has 77 seats: 70 for Fianna Fail, six for the Greens and one for a former Progressive Democrat.
It can further count on the votes of two former Fianna Fail TDs and two independents, according to the Irish Times.
But the support of another three TDs cannot be taken for granted. These include one who was expelled from the party over a stag-hunting bill, one who recently opposed government health cuts and one who left Fianna Fail in 2008 over a different health reform.
Victory for Sinn Fein in Donegal would bring its number of seats in the Dail to five.
The by-election result is expected to be declared officially on Friday evening.