The 1922 Anglo Irish Treaty caused a split in what was then Sinn Fein into factions for and against, which later became Fine Gael and Fianna Fail. Some families were divided by this split, but as a generalisation, Fine Gael was made up largely by a bourgeoisie of larger farmers, small businesses, the professions and those who had a stake in maintaining the economic status quo. Fianna Fail, on the other hand, had a slight preponderance of smaller farmers, the landless, jobless, and those with a lesser vested interest in the status quo.

It could be as simple as a split between oldest sons (who inherited the family farm/business, and their siblings who had almost no prospects.

I would not be surprised if a large majority of those who emigrated to the USA during and after the civil war were those with a lesser stake in the status quo, lesser economic prospects, and disillusion with a political establishment who had traded their dream of a largely secular united Ireland with a much lesser 95% Roman Catholic dominated, conservative, repressive, 26 (out of 32) county state.

However I have never seen any data on the political allegiances of those who emigrated to the USA during this period, so this must remain a conjecture.

Index of Frank's Diaries