The Midland Great Western Railway was looking to modernise its motive power fleet, and turned to their Chief Mechanical Engineer, Walter H. Morton, to design a locomotive which could be capable of handling goods, and if required, for local passenger traffic. The design from the Broadstone Works (Dublin) Drawing Office was for a six-coupled (0-6-0) tender locomotive, and the first appearing from the works in 1921, the last in 1924.
This was the last complete class of locomotives designed and built at Broadstone before closure and transfer of work to Inchicore Works. The only other locomotives to follow from Broadstone were the first of the "kit-built" Woolwich Moguls (GSR Class 372) before this was also transferred to Inchicore.
There was a total of 23 members of the class, the first three were Class F, the next ten (including the five Armstrong Whitworth built locomotives) were Class Fa, and the last ten were Class Fb. All were classified by the Great Southern Railways (GSR) as Class 623, or Class J5. All passed to the Córas Iompair Éireann (CIÉ) on its formation in 1945. Examples of the class survived until 1963, almost to the end of steam on the CIÉ.
In the severe winter of 1946/47 coal was in short supply and, with services disrupted, the CIÉ. took the decision to convert 97 locomotives to oil-burning, the main classes chosen for this being the various 2-6-0s and the 623s, although this was extended to some 4-6-0s.
Locomotives which were converted to oil-burning had a white circle painted on the smokebox and tender sides. A supply of coal from America arrived in March 1947 but it took a further three months to build up stocks before normal services could be resumed.
The MGWR originally painted all locomotives a bright mid-green with black and white lining, but changed the livery to black around 1918. On their first visit to the workshops following the 1925 amalgamation to form the GSR the class were repainted into the standard "dark battleship grey" colours of the new company. Buffer beams were vermilion. Following the absorption of the GSR into CIE in 1945, and CIEs nationalisation in 1950, the only changes which took place were the gradual replacement of the cast cabside number plates with painted pale yellow numerals, and the addition of the light green CIE "flying snail" logo to the tender sides. The grey livery, like on most CIE locomotives, remained until withdrawal in the early 1960s.