The Tain is written in Old Irish, but 12th century is supposed to be Middle Irish. What difference could 200 years make? (That’s a joke, son).
Here’s some source info from Aife in the East Kingdom, about Old Irish spelling and pronounciation.
- Old-Irish Spelling and Pronunciation
- Litreacha na h-Aibítre sa Ghaeilge
- A Beginner’s Guide to Old Irish Pronunciation
- Also recommended, this entire site:Sengoidelc and this online glossary