Mallow was built in 1935 as works number 12662 and was assigned to Mallow where it remained for its working life.
The locomotive was withdrawn from service in 1962 after which it languished at the Mallow factory until it was bought along with two other locomotives (No 1 Thurles and No 3 Mallow) from the original nine. The purchase was made by a Dublin metal dealer on behalf of a private owner who wished to eventually operate them in England.
The three locomotives were moved by rail to Dalkey but the siding on which they were stored was later required for the DART electrification project. The CIÉ then agreed that the locomotives could be stored in a disused bus facility at Broadstone in Dublin but this meant that the locomotives would have to be transported by road to their new home. In order to pay for the transport costs one of the locomotives was sold to a scrapyard in 1977 and it was scrapped the following year.
The two remaining engines ended up at the site of Ballynahinch Junction in County Down in 1978. It was at this point that the owner of the locomotives realised they were the wrong gauge for an English railway. The locomotives remained at Ballynahinch Junction until 1987.
The recently formed Downpatrick & County Down Railway (DCDR) purchased the two locomotives and moved them to Downpatrick.
After reaching the end of its 10-year boiler ticket it was withdrawn from service. The boiler was sent to Whitehead in 2016 for overhaul. By the end of 2018 the locomotive was back in steam, and after a few days of testing returned to passenger service on the last Christmas trains of that year.