An Old Irish Primer compiled by Wim Tigges

This is the best source of an introduction to Old Irish.  The author collaborated with Feargal Ó Béarra in compiling the primer.  An Old Irish Primer is the acknowledged by scholars as the go to book for the Old Irish studies.

We also have A First Old Irish Grammar and Reader compiled by Kim McCone.

We also have a third book on the subject of Old Irish; Old Irish Paradigms compiled by John Strachan.