Mac Cruitín was a native of County Clare, apparently the area of Ennistymon.
Seamus claimed he was sixth in line of descent from Sean, a brother of Aindrias.
Associates included the scribe Michael O Raghallaigh; Brian O Luanaigh (1828–1901), later Professor of Irish at the Catholic University of Irish; John MacHale, Archbishop of Tuam; Eugene O'Curry; William Smith O'Brien.
Mac Cruitín had spent some time in County Kerry, and by his early 20s was working as a schoolmaster.
Mac Cruitín was buried in an unmarked grave in the paupers' plot of Ennistymon Workhouse.