Bartley Clarke and Mary Welch

Bartley (Bartholomew?) Clarke and Mary (Ann?) Welch were my mother's great-grandparents. My information about them is very sketchy, but Bartley, I believe, was born in Ireland in 1841, Mary in 1842. 

Bartley died in 1922, Mary in 1931. I believe they were married in Ireland and immigrated in 1861. Their daughters were Mary Margaret (my great-grandmother) and Kathryn, and ended up living in Tomah, Wisconsin. Margaret's married name was Gleiss. Kathryn's was Christianson. Tomah may be the key to everything, but I sure can't find it.

The Irish communities of Sligo and Roscommon are associated with them, but only as words in one of my grandmother's textbooks. St. Catherine's, Ontario, also is somehow connected with Mary, though I don't know how.

I thought that tracing Bartley and Mary would be the easiest of my ancestors. It has become the most difficult. I don't know where in Ireland they came from, the names and residences of their parents or ancestors (although my grandmother mentioned the name Tighe when referencing Mary's parents), their occupations, or anything else. Completely stymied. And frustrated.

Any help you can provide would be very, very appreciated. Thank you so much in advance.