Agreed - my parents and I all had a hard time picking up on every word, and there were a few lines I didn't get at all. But it didn't really make a difference in our overall enjoyment of the show.
That said, I was interested to know what I missed, so I bought the script. It just came yesterday, so I haven't had a chance to read the whole thing yet, but I'm noticing that there are several lines where it probably wasn't even the accents that were the issue so much as the Irish slang. Even reading it I wasn't 100% sure what they were saying in a couple of places, haha. But I highly recommend gettign the script if you enjoyed the play - I'm already having so many a-ha moments, not even in my understanding of it so much as getting the change to really digest Butterworth's layered writing, something impossible to do in one viewing of the production.