For anyone interested in the above, the following books are useful:
Patrick Maume: The Long Gestation (Dublin, 1999). A wonderful pot-pourri of titbits from the years 1891-1918, yet lacking a clear central argument. Massively researched but poorly presented.
Joseph V O'Brien: William O'Brien and the Course of Irish Politics 1881-1918 (Berkeley, California, 1976). Standard work on O'Brien, but dated.
Sally Warwick-Haller: William O'Brien and the Irish Land War (Dublin, 1990). Good on his early career. Suffers from a lack of objectivity in places, but otherwise a decent read.
Philip Bull: Land, Politics and Nationalism (Dublin, 1996). Great book. Very good on O'Brien's later career from 1898 to 1903. Has also published detailed articles in Irish Historical Studies on the United Irish League.