When researching Kerry families there are a number of resources worth considering. O’Kief Coshe Mang, Slieve Lougher and Upper Blackwater by Dr Albert Casey consists of 15 volumes and is very extensive though mainly covers an area bounded roughly by Newcastle, Tralee, Killarney and Mallow. There is a list of the related Kerry holdings in O’Kief on this website http://www.corkpastandpresent.ie/genealogy/indextookiefcoshemang-kerrymaterial
We hold the full set in our library in London. The beauty of these books are that they are not just indexed by name but by surname with first name e.g. James Fitzgerald, John Fitzgerald, Julia Fitzgerald.
Selections from old Kerry records, historical and genealogical by Mary A Hickson 2 volumes published 1872-1874.
A Handbook of County Kerry History and Biography by the Rev. Sir Henry L.L. Denny published 1923
There is also County Kerry Past and Present: A Handbook to the Local and Family History of the County by Jeremiah King. This is digitised and online on http://www.askaboutireland.ie However I think this book probably does not have enough detail for you on the Fitzgeralds.
Then there is the more recent book Voices from the Great Houses of Cork and Kerry by Jane O’Keefe published by Mercier Press 2013. This is said to include the Fitzgeralds, Knights of Kerry but I am not sure how much detail it goes into.
Have you checked the Irish Genealogist database to see if there has been an article on the extended family in the IGRS journal over the last 80+ years?