The class F was the last locomotive to be designed by the Midland Great Western before its incorporation into the Great Southern Railways. Designed by Walter H. Morton for mixed-traffic duties, it featured a 0-6-0 wheel arrangement and comparatively large driving wheels measuring 5 ft 8 in (1,727 mm) in diameter. The boiler was fitted with a Robinson superheater and had a grate area of 17.3 square feet. Of the 23 locomotives built, 18 were constructed at the MGWR's Broadstone Works between 1921 and 1924, while the remaining five were supplied by Armstrong Whitworth.

Owing to differences between the production batches, the first three locomotives were designated class F, the next ten class Fa, and the final ten class Fb. The latter were distinguished by their raised running boards. In service, these locomotives handled not only freight, commuter, and cattle trains, but also limited-stop mail services. Following the formation of the Great Southern Railways in 1924, they were initially classified as class 623 and later redesignated J5. All passed to the CIÉ in 1945, and when steam traction was finally withdrawn in 1963, members of the class were still in active service.