To help answer your first question, I would recommend reading Gaelic and Gaelicized Ireland in the Middle Ages, 2nd ed. by K. W. Nicholls (http://www.worldcat.org/oclc/471778201) and A New History of Ireland, Volume II and III (http://www.worldcat.org/oclc/495293791 and http://www.worldcat.org/oclc/299242437 respectively). As for your second question, I am not an expert enough to answer because I tend to study the earlier period before the Norman invasion.