Micheál was born in Newtown, Waterford in 1871 but moved to Dungarvan with his family when he was ten years of age. Irish was still spoken in Dungarvan at this time and this influenced him greatly. He trained for the priesthood and was ordained at Maynooth in 1895. Ó Síocháin continued his studies at Oxford where he took his M.A. in 1897 and then at Bonn where he graduated with a PhD in 1900. He became a very distinguished Classical and Irish scholar and was appointed Professor of Classics at Maynooth. In 1919 he became president of that College.

Ó Síocháin, together with Fr. Richard Henebry, founded Ring College in 1906. Their aim was to provide instruction through Irish for the school year, and also instruction during the summer months. In the first year there were only half a dozen students but by the following year this had risen to 60. The number of students at the college in July and August of 1909 was over 200. Ó Síocháin published a number of books, including Seanchaint na nDeise, in 1906. He was appointed assistant Archbishop of Sydney in 1922. On 1 June 1934 he resigned from the Coadjutor Archbishopric of Sydney and returned to Ring. In 1938 he was appointed Count of the Papal Court, with the right to assist at the Papal Throne and other privileges. Ó Síocháin died in Ring in 1945 and was there laid to rest.