The Córas Iompair Éireann (CIE) ordered 15 of these 4-axle 960bhp diesel locomotives from General Motors locomotive building Electro-Motive Division (EMD) in 1960. The locomotives were based on the builders' end cab switcher designs, equipped with an 8-cylinder 567 series engine and riding on trucks suitable for 'road' service up to 125kmh/77mph. Designated class B and numbered B121 to B135 the locomotives entered service in 1961 painted in a grey and yellow livery, quickly replaced by the familiar black & tan scheme.
The locos displaced steam engines entirely from mainline services, working mostly to Cork, Sligo and Galway. Although initially run in both directions an accident early in the locomotives lives resulted in 'cab leading' being the normal operating mode, giving much better visibility. The class was fitted for multiple working with other class 121 and class 141/181 locomotives in the 1970s and latterly often worked passenger trains in push-pull mode. 126 to 129 (at least) also received engines upgraded to the later EMD 645 cylinder size rated at 1,100bhp, as fitted to the 141/181 class locos.
The complete class remained in service until 1986 when 125 was withdrawn following an electrical fire, the loco also having sustained accident damage previously. The loss of their main push-pull passenger work in the mid-1990s saw most of the class stored by 2000, though even the damaged 125 wasn't cut up until 2002. By 2005 only numbers 124 and 134 were still active, these two locomotives being withdrawn in July 2008, though it seems they last been used in March. Both were preserved, 124 by the Irish Traction Group on the West Clare Railway and 134 by the Railway Preservation Society of Ireland (RPSI) who plan to return the locomotive to mainline service as part of their railtour locomotive fleet.