8 June 1780, William Crolly was born in Ballykilbeg, near Downpatrick, where he attended a classical school, conducted by Reverend. Mr. Nelson who was a Unitarian Minister.
In 1901 he obtained first place in dogmatic theology and was ordained priest. For the following six years he lecured in logic, metaphysics, and ethics and in 1812 he moved to Belfast. In 1825, he was appointed Bishop of Down and Connor and was instrumental in persuading the Vatican to change the episcopal parish from Downpatrick to Belfast. During his time there he built a large church is almost every parish and founded St Malachy's College.
In 1835 he was appointed to the archdiocese of Armagh and in 1838 he founded St Patrick's College.
He is best remembered for his work on the foundation of the Armagh Cathedral, which was not completed until 24 years after his death on 6 April 1949, and he was buried in the centre of the choir of the unfinished cathedral.