There are three big local events that I know of this summer.
First up, July 15-17 is the International Maritime Music portion of the Bay City Tall Ships Celebration. While the focus is on Maritime music, a lot of the groups also have a strong Celtic bent, including the local group Whisky & Water.
Then in August there are two events the same weekend. August 25-28 is the Midland Folk Festival. I don’t know what all will be there yet, but if nothing else I have volunteered to teach “Irish Jigs of Vincent Broderick” on Friday.
While that’s going on, on Saturday the 27th Saginaw hosts the Great Lakes Gathering, which is all Celtic music all the time. I’m not seeing a comprehensive list of who is playing there this year yet, but there should be a lot of great groups.