I was just browsing around and found that the mandolin site CoMando has over 400 MIDIs. Unfortunately not many fall in the "Celtic" category but there are some. Most are bluegrass and old time with a sprinkling of various other categories. Some are mislabelled. Off to California was called old time. If you play mandolin there are over 600 tabs there.
http://www.co-mando.com/
Steve