A number of points : firstly, a drum is a drum and can be used for many types of music - it depends on how it's played. For example, I used to use one very effectively for Kentucky Running Set with more syncopation than would be appropriate for Irish. Secondly, drummers are musicians - just look at Evelyn Glennie, and Easy Club's Jim Sutherland who played cittern as well as bodhran and wrote some great tunes. Personally, I would never join in with a musician on stage unless specifically invited to so. I'm even reluctant to join in with chorus songs until the singer has indicated that they want people to participate.