Although prototype diesel locomotives ran in Britain before World War II, the railways of both the Republic and Northern Ireland changed over much more rapidly from steam to diesel traction than those in Britain, due to the island's limited coal reserves and (in the Republic) an ageing steam locomotive fleet.
Northern Ireland operated several diesel shunters as early as the 1930s. CIÉ's first diesels consisted of five shunters built by CIÉ in 1947/48. The initial two diesel mainline locomotives were also built in Inchicore, in 1950/51, and fitted with Sulzer engines and MV traction equipment. This was followed in the mid 1950s with a large order from Britain fitted with Crossley engines, with notably poor results. From the early 1960s, locomotives with more reliable engines from General Motors Electro-Motive Division were adopted. In the late 1960s the Crossley engines were replaced by EMD 645 units in a major re-enginging programme. Since the early 1960s all new locomotives on the two Irish rail systems have been purchased from EMD, with the exception of three from Hunslet Engine Company of Leeds, England, for NIR in 1970.