There are/were lots of Larkins in Ireland! Many of them in the Galway-Roscommon area. There are Larkins in my family; I don't know if your Michael is one of them. What town did he settle in? Did he live with other people named Larkin, in the censuses? Often people came to a place they already had relatives in, or more relatives came to be with them. There could be more info in the records of these people if not in Michael's.