During following summers he returned to Donegal and reorganised the Volunteers in that county. By October 1916, he'd been released, whereupon he rejoined the Volunteers and once again resumed his studies at UCD.

Following the surrender of rebel forces, Sweeney was interned in Frongoch internment camp in Wales. Sweeney was active during the events of the Easter Rising, where he fought in Dublin's Liberty Hall and later the GPO. In 1915 Sweeney was sworn into the Irish Republican Brotherhood by Pearse. By September 1914 he was a student in University College Dublin and he had transferred to Pearse's unit in the Dublin Brigade, who styled themselves 'Pearse's own' and had many former students of Pearse amongst their number. Pearse solidified Sweeney's nationalist beliefs, and in 1914 Sweeney joined the Irish Volunteers himself.

Enda's School in Rathfarnham, where Patrick Pearse was Headmaster. He received his secondary education at St. He came from an Irish Nationalist family his father had been jailed during the Land War as well as being a president of the local branch of the Ancient Order of Hibernians and a founder of the Irish Volunteers in Donegal. He was the son of general merchant John Sweeney and Margaret O'Donnell. Sweeney was born and raised in Burtonport, a town in The Rosses, a district in the west of County Donegal.

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