The Córas Iompair Éireann 121 Class was a railway locomotive which was manufactured by General Motors Electro-Motive Division.

[3] The locomotives proved an immediate success, with low maintenance and high availability, and led to further orders from the same supplier starting with the 141 class.

[1] From the early 1970s onwards several locomotives of this class dropped the "B" prefix from their fleet number when re-liveried.

The layout of the cab was quite different from the other conventional CIÉ diesel models of the time, with the controls to the side of the driver, rather than the front.

Although originally fitted with an EMD 8-567CR engine of 960 horsepower (720 kW),[2] all were later fitted with 645 type "power packs" (piston & liner assemblies) for parts standardisation, while at the same time keeping their original power output for reliability reasons.

In 1994, a railcar revolution had begun, and the push pull carriages were later re-deployed to inter-city duties with the 201 Class locomotives.

The first member of the class to be withdrawn was 125 in March 1986 following an electrical fire, though it had been extensively damaged in an accident twelve years previously.

The other surviving member of the class, locomotive B134, is owned by the Railway Preservation Society of Ireland (RPSI), and is undergoing restoration.

On the 7th of May 2022, 134 was displayed at the Inchicore Works during the opening day's 175th anniversary - a commemorative plaque was unveiled upon the locomotive, much like 141 Class number 150 during 1996.

This was replaced within a year by a black / tan (“golden brown”) colour scheme with a thick white band similar to the Cravens coaching stock, delivered in 1963.