Doug:
I would vote for 2 rounds on the Old. Glashedy is OK, and certainly is better than some other modern links courses that seem too manufactured for my tastes, but it is not as enjoyable as the Old, which is a very good links course.
If you are looking for a bonus round on your way up to Ballyliffin, consider a quick 9 or 18 at NorthWest GC in Buncrana. We saw NW beside the road on our way up to Ballyliffin and stopped in for a quick 9 (played the front). Not a great links by any means, but a number of fine holes and a lot of fun. Perhaps it was because we had no expectations, but several members of our group enjoyed NW more than Glashedy and either course at Rosapenna. Like I said, we only played the front 9, so for all I know the back 9 is a real clunker.
Whatever you decide, you are heading to one of the most beautiful golfing regions I have ever seen.
Mike