Shiell was a member of the Ó Siadhail family of Kingdom of Uí Failghe, who were prominent County Offaly and County Laois in the late medieval/early modern era. Shiell was a member of the Franciscan order, and became the guardian of the Franciscan friary at Killeigh, County Offaly, in 1693. In 1698, eight individuals signed a document acknowledging that they had received chalices, pyxes, cups, an oil box, ciborium, a bell and vestments of the friary for safekeeping. Michael Shiell was one of the eight signatories, as was one William Shiell, who received at least three of the items. Krasnodebska-D'Aughton states that "both were probably members of the Ó Siadhail family, who had long associations with the Franciscans in the midlands."
- "Franciscan Chalices 1600-50" by Malgorzata Krasnodebska-D'Aughton, p. 301, "The Irish Franciscans 1534-1990", ed. Bhreathnach, MacMahon and McCafferty, Four Courts Press, 2009.