Well the death of Dev has huge effects on how both the treaty is debated, and how the peace conference is set up to begin with. It probably has huge affects on the Civil War itself. So it would have a significant change on how Collins was both viewed and acted. As Director of Intelligence for the IRA, and the President of the IRB he would have even more power as a leader in the independence movement, and would have a decent chance at becoming the Taoseach as his position in the IRB made him according to them the President of the Irish Republic. If he had survived, of course we would be typing about what shape Ireland would be in if Dev had survived. We're all about discussing alternate history. Unless you mean that the site would be butterflied away, but not even I would give the Big Fella that much credit.
Certainly I understand your motivation, I was just curious to the possibility as it seems a decently plausible one that Collins is even more prominent with Dev out of the picture. Also in this case I don't think Collins is on the treaty delegation, so that brings up another bunch of potential butterflies. I do think though that Collins would pursue the same route Dev eventually did in regards to the treaty, and that eventually Ireland would gain full status as a republic (perhaps sooner ITTL). Maybe this is facilitated by a much less severe or non existent Civil War, and more economic development spurred by Collins and his successors in order to quicken the day when full independence is declared. It be interesting to see a Collins government's relationship with Ulster as well.