In 1914, Moylan joined the Kilmallock division of the Irish Volunteers but left in 1914, when his apprenticeship finished and he moved to set up a business in Newmarket, County Cork.

[3] He had risen to the rank of Officer Commanding the Cork No.2 Brigade when he was captured and interned in Spike Island in May 1921.

He was Director of Operations of the Anti-Treaty forces and a member of the IRA Executive, holding the rank of commandant-general.

Moylan lost his Dáil seat at the 1957 general election, but was nominated by the Taoiseach to Seanad Éireann as a senator later that year.

Speaking at an event commemorating the 50th anniversary of his death, Brian Lenihan Jnr suggested that Moylan was "one of the most outstanding military leaders in the War of Independence".

[6] Moylan is mentioned in the Irish folk ballad "The Galtee Mountain Boy", along with Dinny Lacey, Dan Breen, and Seán Hogan.