Thomas McDonagh was born in Cloughjordan, Co. Tipperary, in 1878. Due to an interest in the Irish language, he joined the Gaelic League. He helped to found St. Enda's School with Pádraig Pearse and became the school's first teacher. Later, he became Professor of English at the National University.

McDonagh joined the Irish Volunteers in 1913. He was co-opted onto the Military Council of the Irish Republican Brotherhood weeks before the 1916 Rising, and was one of the signatories to the Proclamation of Independence. He commanded the Second Battalion of the Volunteers, which occupied Jacob's biscuit factory during the Rising. McDonagh was executed for his part in the Easter Rising on the 3rd of May 1916.