The class A were the heaviest locomotives in Ireland at the time they entered service. Their job was to haul heavy express trains between the east and west coasts. Due to their high axle load, they could not be used everywhere. The first locomotive was rebuilt as early as 1916, with a superheater and larger cylinders. Due to differences, the rebuilt locomotives were designated class A1 or As. With the exception of one locomotive, all the others were only withdrawn between 1954 and 1959.