Micheal Martin has ruled out a power-sharing premiership with arch-rival Enda Kenny just days before Irish voters are expected to return a hung parliament.
The Fianna Fail chief dismissed as impractical any deal to share the Taoiseach's office with the Fine Gael leader over the lifetime of the next government in a so-called grand coalition of the historic rivals.
"It's not a consideration," he said.
Former Fianna Fail minister Conor Lenihan suggested the two main parties could end their civil-war schism and come together on the basis of a rotating Taoiseach to form a stable government next week.
Latest opinion polls show little chance of the current Fine Gael/Labour coalition being returned to power.
A massive divergence in voter preferences, with Independents and smaller parties looking likely to suck up well over a quarter of the vote, has cast much doubt over the future make-up of an administration.
Mr Martin said he didn't see the prospect of a rotating Taoiseach "as a practical possibility".
One of the last major polls of the election campaign shows Fine Gael remain the largest party, on 30% of the sample vote, while their junior partner Labour has slipped to 7%, down 1%.
Fianna Fail has climbed to 20% - up 2% - in the popularity rankings since the last survey by the same pollsters Red C last week.
Sinn Fein dropped back 1% to 15%.
Independent candidates and other smaller parties retain a huge share of the vote at 28% overall.
The Social Democrats are on 4%, Anti Austerity Alliance/People Before Profit are on 3%, the Greens on 3% with Renua on 2%.
The pollsters surveyed 1,002 voters between Thursday and Sunday, the last weekend of canvassing.
On the back of the latest poll, bookies have further slashed the odds of a Fine Gael/Fianna Fail coalition - making it the odds-on favourite outcome - despite repeated denials from both camps that a coupling was in the offing.
Labour's public spending minister Brendan Howlin said the parties would stand accused of a breach of faith if they came together to form the next government.
"Both have said emphatically that they won't join up to now," he said.
"If Fianna Fail go in as the junior party with Fine Gael, ultimately it is the end of Fianna Fail."
As Fine Gael launched an online video campaign urging voters to allow them to "keep the recovery going", a former International Monetary Fund chief said the party played no role in the Irish economic bounce-back.
Ashoka Mody, the IMF's mission chief in Ireland as it slid into an embarrassing international bailout six years ago, told the Sunday Business Post that the country's low tax regime was responsible for the recent economic boost.
"The tax system attracted some real investment into Ireland and that helped propel the Celtic Tiger. It was a reasonable way to get the economy jump-started," he said.
"And some part of the recent recovery has been helped by the legacy of those earlier efforts.
"To the extent that this has happened, it has nothing to do with the Troika programme or the efforts of the current government."