Originally published as Gaelic Personal Names, this is a new edition. It is the very finest book on Irish names available. It is a dictionary of irish given names, with modern and ancient forms, derivations, origins, and period citations. Oxford English Dictionary, Oxford University Press, 1971. A surprising number of bynames can be documented from this dictionary.
Entries give various spellings with dates, back to the earliest written citation.
The best surname dictionary anywhere. Although this work concentrates on English surnames, it often includes Scottish, Irish, and other surnames. Each entry includes citations as far back as possible, with dates, in as many forms as possible. The introduction is a fabulous essay on surname formation, applicable to any European language. This is a standard SCA source, found in most libraries.