One of the things that sets The Green Flag apart is the fact that it covers a longer period than many of the histories of Irish nationalism that you will find. D. George Boyce's Nationalism in Ireland is another crucial text for this topic. Boyce provides a survey of the myriad branches of Irish nationalism including its sixteenth-eighteenth century precedents. Most other scholarship on the topic tends to focus on specific periods or regions (Ulster is very popular due to the Troubles). Some really good work on this front has been done by Paul Bew in his books, especially Ideology and the Irish Question: Ulster Unionism and Irish nationalism 1912-1916. This one covers the formation of the various paramilitary factions associated with nationalism and unionism. Bear in mind that you will probably want to look into some scholarship on Irish unionism because many scholars define it as a form of nationalism (though not the dominant form in most of Ireland). There is a decent listing for Nationalism and Unionism on the bibliography at UNE.

I'm not a big fan of Kee. It is pretty out of date and is more a popular than scholarly work. In addition to Boyce and Bew try Richard English Irish Freedom: the history of Irish nationalism