In "The Week In Politics"'s report on the Constitutional Convention's meeting on electoral reform, Adrian Kavanagh provided analysis on how the 2011 election result would have looked if held under FPTP, and the differences are quite staggering:
Fine Gael 114 (36% of vote/69% of seats)
Labour 32 (19/19)
Independents & Others 11 (18/6)
Sinn Féin 6 (10/4)
Fianna Fáil 3 (17/2)
So, Fine Gael would likely have secured 10-15 years of single-party government, Labour would have secured a left-right divide as leaders of the opposition, and the two republican parties would have been looking at a merger to remain numerically relevant.