Welch/Welsh/Walsh from Ireland to Michigan

I am researching my maternal GGgrandpaents who were Thomas Welch (seen spelled also as in subject line) and Julia E. Shehan/Sheehan.I beleive Thomas came to the U.S. in 1848 with his father Patrick and 14 other family members.It appears they went to Michigan and joined many other family members in Washtenaw, Co.Does anybody have this family in their database, or knowledge of what drew these Irish immigrants to Michigan? Thomas removed to Oil City, Venango Co., Pa probably in the 1860's where in March of 1864 he enlisted in the Civil War.Grateful for ideas and advice.