Collins was born in Woodfield, Sam's Cross, near Clonakilty, County Cork, on 16 October 1890, the third son and youngest of eight children. His father, Michael John (1816–1897), was a farmer and amateur mathematician, who had been a member of the Irish Republican Brotherhood (IRB) movement. The elder Collins was 60 years old when he married Mary Anne O'Brien, then 23, in 1876. The marriage was apparently happy. They brought up eight children on a farm called Woodfield, which the Collins family had held as tenants for several generations. Michael was six years old when his father died.