* Senior Fine Gael, Fianna Fail MPs raise doubts on alliance
* Leaders will only consider options after final results
* Parties fear giving boost to left-wing Sinn Fein
* Weakness in Irish bond expected, "Brexit" risks loom
By William James and Padraic Halpin
DUBLIN, Feb 28 (Reuters) - Ireland's political stalemate following an inconclusive national election deepened on Sunday after senior figures in the two biggest parties expressed opposition to forming together what looks like the only viable coalition government.
With vote counting still underway after Friday's poll, the heavy losses suffered by Prime Minister Enda Kenny's Fine Gael continued to mount. However, its old rival, Fianna Fail, has so far outperformed forecasts, making an unprecedented coalition between the two centre-right parties the most obvious option.
But their mutual enmity, dating back nearly a century to the Irish civil war, persists, prompting some to speculate that the only way to break the deadlock might be a second election that would put financial markets on edge.
"I wouldn't favour a government with Fine Gael," Fianna Fail's enterprise spokesman Dara Calleary told national broadcaster RTE late on Saturday.
"Any programme for government would have to be put before our members and I certainly can tell you the Fianna Fail people here (in my constituency) are overwhelmingly against any coalition."
Senior members of Fine Gael, which won 25.5 percent of first preference votes and is set to remain the biggest party, though with far fewer seats than predicted, were equally reluctant.
Health Minister Leo Varadkar, widely seen as the strongest contender to eventually succeed Kenny, said a grand coalition was not something he favoured, while Finance Minister Michael Noonan raised the prospect of a second election "very shortly".
"I just don't think it's going to happen, I don't think Fianna Fail will go in as a minority party with Fine Gael," Fine Gael PM Eoghan Murphy told Reuters.
Two other Fianna Fail frontbench members opposed a coalition with Fine Gael. One warned it would boost the left-wing Sinn Fein, the former political wing of the Irish Republican Army (IRA), by turning it into the main official opposition.
"GRAVE INSTABILITY"
Leaders of both parties insisted they needed to wait until the vote counting ends on Monday before considering their options and many on both sides conceded a solution was unlikely to be found by the time parliament resumes on March 10.
That would echo recent elections in Portugal and especially Spain, whose ruling centre-right party failed to win re-election in a December poll. The country remains without a government.