Stynes was born in Newbridge, County Kildare, and attended Newbridge College, where he first played Gaelic football and hurling. He moved to Dublin after World War I to find work. He was sworn into the IRA in 1920 by Seán Lemass, joining "C" Company, 2nd battalion, Dublin brigade. He was stewarding in Croke Park on Bloody Sunday 1920, while carrying concealed guns for the IRA. When British security forces raided the ground, he dumped the guns and escaped over a wall. He participated in the burning of the Custom House in 1921.