I have a question.

Why is the Fine Gael / Labour coalition considered to be, not only the only game in town, but the best option for the country?

Did you notice that Eamonn Gilmore said, more than once in post election interviews, that the norm is for the largest party to lead a government with a small party, and for the second largest party to lead the opposition?

Why is a coalition between fine Gael and Fianna Fail not being considered?

If it is only because Fine Gael have said that they will not share power with Fianna Fail that is now a nonsense - things have changed, changed utterly: when those statements were made none predicted that FF would be reduced to the powerless rump that it is now.

There would be benefits for Fine Gael: they would have to forsake fewer and less central cabinet seats than they will have to surrender to Labour.

But most importantly, there would be an advantage for the citizen and the country: we would have a strong opposition in Labour: if Fine Gael and Labour coalesce the government will have an enormous majority and the opposition will be weak and fractious.

It does not follow that a coalition with a large majority ensures stability, especially when the minor partner is also large and strong.
With a majority as big as Fine Gael and Labour would enjoy, it would be almost impossible to maintain discipline: malcontents and mavericks, within either party, could feel free to break ranks knowing that their action would not threaten the government.