After reading a little I saw that some members of the IRA opposed the Anglo-Irish treaty of 1921. However it didn't mention why they opposed it. So I was just wondering why they would oppose the treaty to make Ireland independent? Isn't that what they were fighting for?
The bottom line was that the treaty didn't make Ireland independent enough for them. The Irish Free State was still to be a part of the British Empire, with an oath to be faithful to the King. Remember that the "R" in IRA stands for "Republican."
The other slight catch was that it didn't guarantee that Northern Ireland would constitute a part of the Irish Free State. It would be, for them, the whole island of Ireland, or nothing.