Which side would you support during the Irish Civil War?

[Proud Lieberal from Northeast]

Go.

[dialectical fetishist]

Pro-treaty I guess.

[Is Totally Not Feeblepizza.]

Yeah, probably pro-treaty.

[wormyguy]

Anti-treaty, though that would probably be just supporting without fighting.

[Jack Vance RIP]

Emigration.

[Frodo]

Anti-treaty.  No way would I be pledging allegiance to a foreign monarch, even if he or she is there only in a symbolic role.  I would demand nothing less than complete and irrevocable independence. 

[patrick1]

My relatives/ancestors? were anti Treaty so Id assume that. 

To me it has always been a realist vs idealist break.  The anti Treaty position was really untenable from a practical standpoint. 

[Formerly Californian Tony]

I'd simpathize with the Anti-treaty but I wouldn't care enough to fight.

[Michaelf7777777]

Pro-treaty by far the best Realpolitick position as the monarch did not have much power anyway.

[Communists For McCain]

I'd simpathize with the Anti-treaty but I wouldn't care enough to fight.

[Dr. Cynic]

I wouldn't be willing to fight, but I would've been anti-treaty.

[Jbrase]

Anti-Treaty

[rob in cal]

Pro-treaty, take a sure 75% win, don't risk the overall goal of Irish independence.  (had the anti-treaty forces looked like winning, wouldn't that have meant the possibility of British intervention?)

[The Mikado]

Emigration.  (Seriously, 1920s Ireland)

[Oakvale]

Anti-treaty, but, seriously, what Mikado & Gully said.

[pillage the village, trash the scene]

Anti-treaty and I would fight.

[Swedish Cheese]

Pro-treaty of course. It was a good, sensible compromise and the most reasonable option at the time.

[Jack Vance RIP]

Pro-treaty of course. It was a good, sensible compromise and the most reasonable option at the time.

There was nothing 'sensible' about the anti-treaty side:

"People may have to die in this country and may have to die through starvation" - actual quote from the then (1924) minister for Industry and Commerce on the nature of his governments' economic policies.

[Jerseyrules]

Pro-treaty of course. It was a good, sensible compromise and the most reasonable option at the time.

[politicus]

Anti-treaty because of the border issues.

The treaty left catholic majority counties Fermanagh and Tyrone  in Northern Ireland. Derry and Southern Armagh also had catholic majority.
Fighting might have been hopeless but the treaty was simply to unfair. 

[Vasall des Midas]

Depends where I'd be from and who I knew and liked, don't it?

[Jack Vance RIP]

Pro-treaty of course. It was a good, sensible compromise and the most reasonable option at the time.

Why "the most reasonable" option?

Btw, Lewis' answer is the best one.

[Peter the Lefty]

Anti-treaty, but I'd also be afraid of an Irish Republic due to the (eventually realized) possibility of the "Catholic social doctrine" having control over the Irish legal/political system.