The Old Shillelagh is one of my favorite downtown bars. They have a great selection of beers (although not a good as the DBC) and they have a great, fun, and laid-back atmosphere. The place is usually packed before and after Tigers games since they run a shuttle (even though it is within walking distance). The bartenders are pretty nice folk and the prices are ok.
I've never been upstairs at the Old Shillelagh, but I heard that it gets pretty roudy on Friday and Saturday nights with Irish music and people singing. Next time I go, I plan to go upstairs and check out the happenings.
If you're the kind of person who likes "fake" Irish bars, like the Claddaugh chain of bars, then the Old Shillelagh is *not* for you. If you're looking for good time in downtown Detroit this is a great place to start (or end) and evening.