Yes, Birr castle is definately worth a visit. The telescope itself has been almost restored to its original glory but is not yet operational. Last time I was there in October I heard that they hope to have it up and running sometime next year.
The other place in Dublin is Dunsink observatory
www.dias.ie. I understand that there are observing sessions open to the public from time to time but i have not been there in a great many years.