Mainly it was due to the British screwing everything up. There was an increasing widescale perception that Home Rule would not be implemented when the war was over, but things weren't really bad until 1916.
The British completely overreacted to the Easter Rising, which, as I've said before, sets the bar for incompetent revolutionaries.
First they overreacted in a military sense, but even then the rebels were viewed in Ireland primarily as sauerkraut-sucking agents of the Kaiser, which they practically admitted to as much in their manifesto.
However, the British completely ignored how unpopular the Rising was in Ireland, and decided some good harsh political repression was neccesary. They rounded up almost every major nationalist in Ireland, regardless if they were republicans or monarchists and put them in concentration camps in Wales.
This had two effects: First it immediately pissed off anyone seen to actually represent Irish interests. If they were monarchists before they went to Wales, they were almost certainly republicans afterwards.
And second, anyone who wasn't sent to a prison camp (Redmond) was immediately delegitimized, because he couldn't be much of an Irish Nationalist if the British Army didn't think so.
Worst of all, this didn't actually do much to hurt the IRB itself. They were a secret society that adopted a policy of infiltration a while ago and so their members were by definition, not prominent. The removal of major nationalists probably helped with IRB efforts to subvert the GAA and so forth (I think this is probably the largest effect control of a sports club has had on history).
Violence picked up again as the nationalists started leaving the camps and the IRB got a new breath of life. This of course brought the Black and Tans in. This not only completely alienated the population, it also removed the greatest threat ever held over Irish independence: that without British Rule, there would be a civil war between Catholic and Protestant.
But by the 1920s it certainly looked like a sectarian war was already happening, and independence was the only way out.
So to summarize, the change in leadership happened as a result of radicalization. But, I wouldn't say there was all that much radicalization. The issue was that Home Rule had been shown to be disastrous and by that point it was clear that such proposals were essentially non-starters. Independence offered the only real solution.