I was wondering if someone would come and say "but some people say "thá"!". But just in case, I didn't mention it in my answer to keep it simple :-)
One may say thá and athá are common in several Munster dialects, but not all.
Actually in "athá", the lenition of "tá" isn't caused by the relative particle "a", it is a permanent lenition. Even without "a" before, people say "thá" (in the dialects where it does exist of course).
So in these dialects, everytime you'd expect "tá" they say "thá" and everytime you'd expect "atá" they say "athá".
It's not a grammatical lenition, like.
quote:In my gaelic guide (patrick le besco "parlons gaélique")
be careful, this book is, unfortunately, full of mistakes...