Don't forget to visit the web pages of college and university libraries. Many of these libraries have digitalized many of their collections, and you can download source materials without leaving your home. Many belong to the WorldCat system described in my earlier posting today, or they will otherwise interlibrary loan materials.
Irish family history researchers should explore the holdings and collections of some of the universities with an Irish connection or tradition. For example, Villanova University houses the McGarrity collection, which is a treasure trove for those of us whose genealogical research includes Ireland's 20th century struggle for independence (and also for Co. Tyrone researchers). I know Irish genealogy researchers who have found valuable materials at Seton Hall's library.
Don't forget, too, the vast online digital collection of Google Books. Not only can you read a book online as a pdf file, but you can search it using the find function. Under "links" below, I am linking to an example I found through Google Books of an invaluable Co. Kilkenny source, a Report on the Manuscripts of the Marquis de Ormonde.
Report on the Manuscripts of the Marquis de Ormonde
Villanova University Library
Seton Hall Library
Google Book Search