The Collins Barrcaks is the oldest continuously occupied barracks in the world and since 1997 has been the site of part of the collection of the National Museum. This part of the National Museum the collection of decorative arts and economic, social, political and military history. There are permanent exhibitions as well as traveling ones. The permanent exhibitions include ceramics, silveware, furniture and fashion. It is well worth visiting and is one-third of the National Museum.
The Natural History Museum on Merrion St. with the Kildare St. Museums make up the other 2/3 of the National Museum. I didn't have time to visit the Natural History portion but have heard it is very nice.