The following books are extremely useful and are all used when writing articles for this website. Without these books this website couldn't happen so thank you to all the authors. This is a growing list:
The Archaeological Inventory series published by Wordwell Books is a must have for sites not in the guide books. Published county by county they cover every site in their respective county. But be warned, they also cover sites that are no longer in existence so it is worthwhile to learn how to use the book properly or you could end up in a field looking for something that is no longer there!
O'Croinin, D, A New History of Ireland, 9 vols, Oxford University Press, Oxford, 2005. Although expensive these books are extremely detailed and can't possibly leave you wanting to know more. Volumes 1, 2, and 3 take you up to 1691 which should cover nearly all the site in the country.
McCarthey, G, The Forgotten Heritage of Kildare, Nonsuch Publishing, Dublin, 2006.
O'Kelly C and M.J, An Illustrated Guide to Lough Gur, Co. Limerick, Houston, Cork, 1978.
Fitzpatrick, E and O'Brien, C, The Medieval Churches of County Offaly, Duchas, 1998.
Herity, M, Rathcroghan and Carnfree: Celtic Royal Sites in Ireland, Na Clocha Breaca.