The largest express passenger locomotive in Ireland for a short while after introduction they were used on the MGWR's flagship services to Galway with most surviving until the 1950s albeit on less prestigious work.

With Class K the limits of the 2-4-0 were being approached and the rival Great Southern and Western Railway (GS&WR) had been using 4-4-0's not only on their main Dublin-Cork but secondary routes also and were at the beginning of the 1900s introducing a further generation of larger 4-4-0s.

[3][4] Martin Atock of the MGWR was known to be resistant to bogies but perhaps influenced by his deputy Edward Cusack in 1900 at the end of his service organised rebuilds of older 2-4-0 locomotives to a small 4-4-0.

When Cusack succeeded Atock he chose to quickly follow through with six large 4-4-0s that to were become known as Class A for the MGWR's premier passenger trains and which were the largest in Ireland for a short while after their introduction.

124, which had been upgraded to As standard in 1916, was involved in an Irish Civil War incident at Streamstown south of Mullingar in 1923 which required it to be totally rebuilt with shortened frames and re-designated A1.

646 was again rebuilt in 1936 with a type A boiler, as were No.s 545 (127) and 546 (128) in 1937, the resulting locomotives having the maximum axle load reduced to 17 long tons 10 cwt (39,200 lb or 17.8 t)[4] The introduction of the A class in 1902 enabled 16 minutes to be cut from the prestige limited mail[a] service from Dublin Broadstone to Galway, the coaching stock also being upgraded to include bogie coaches at the same time.

[3] Oswald Nock in his book "Irish Steam" comments the MGWR schedules did not tax these locomotives and indicated a run of an average speed of 51.4 miles per hour (82.7 km/h) for the 38.6 miles (62.1 km) from Killucan to Ashtown with a load of 290 long tons 0 cwt (649,600 lb or 294.7 t) superheated Mercuric.

There high axle load limited them from most branch lines and they were mostly used for auxiliary goods (freight) for which they were not efficient.

[3] The blue paint was however found not to be durable and faded easily after a few years and was replaced with green, albeit of a lighter shade than the MGWR had used hitherto.