For a long time Fianna Fáil didn't form coalitions at all. Fine Gael, on the other hand, has always been the second-largest party in the Dáil, and as a result has always had to form coalitions to take power. Given that Labour has always been clearly the third-largest party in Ireland, they are Fine Gael's traditional coalition partners. Moreover, the Progressive Democrats were originally Fianna Fáil dissidents.
Why did FF not form coalitions? For any sort of non-ideological, dare I say "American-esque", party, holding power by any means necessary would seem like a logical modus operandi. Lacking ideological convictions & being unwilling to cooperate with other parties seems like a recipe for irrelevancy.