The 4th Battalion consisting on the day of approximately 120 Volunteers (out of a normal strength of 500) under Commandant Eamonn Ceannt occupied the South Dublin Union, located to the south of the Liffey, two miles to the west of the GPO. The Union was built as a workhouse in the middle of the nineteenth century; in 1916 it housed about 3,200 of the poor and elderly and a large staff of doctors, nurses and ancillary workers. In the circumstances, its choice as a stronghold during an insurrection was surely inappropriate. Nurse Margaretta Keogh was accidentally shot dead in the course of the fighting.

The South Dublin Union was in a strategic position as it overlooked Kingsbridge (Heuston) railway station to the north and controlled the route from Richmond Barracks and the Royal Hospital (military headquarters) leading to the city centre. It consisted of a complex of buildings serving a variety of functions and included living quarters, an infirmary, a hospital and churches; it was laid out in streets, alleys and courtyards and was set in fifty acres of green space surrounded by a high stone wall. The main cluster of buildings opened onto James’s Street; there, Ceannt established his headquarters in the night nurses’ home. He deployed his men at appropriate points, displaying considerable tactical judgment. Ceannt also occupied three outposts, assigning a captain and about twenty men to each: Captain Seamus Murphy to Jameson’s Distillery in Marrowbone Lane to the south east; Con Colbert to Watkins’ brewery in Ardee Street to the east; and Captain Thomas McCarthy to Roe’s Distillery in Mount Brown.

The most intense fighting was on Monday and Tuesday, during which military occupied and remained in possession of parts of the complex. On Thursday large forces of military pressed home the attack, but were forced to withdraw late that evening, both sides having suffered losses proportionate to their numbers. By that stage General Lowe had decided to concentrate his attention on the GPO and the Four Courts. News of the general surrender order did not reach Ceannt until Sunday.