I know I'm a little late replying but the Union barracks was more than likely the old Union Workhouse on John's Hill in Waterford. There were only two proper barracks in Waterford at the time, the Infantry Barracks in Barrack street and the Artillery Barracks at the end of Cannon Street. Several other former workhouses around the country were used as barracks at the time including the ones in Carlow and Limerick. The old British barracks in Waterford were damaged during the siege of Waterford in July 1922 so the workhouse was probably used instead.