The Córas Iompair Éireann 301 Class locomotives were the first diesel locomotives used on the CIÉ network, this class of 5 being built between 1947 and 1948 by the company for shunting use, particularly in the railway yards on Dublin's North Wall. They were a six coupled (0-6-0 wheel arrangement) locomotive, fitted with a Mirrlees TLDT6 engine of 487 horsepower (363 kW) with diesel-electric transmission via two Brush traction motors. Unusually, they lacked train vacuum brakes, although air brakes were provided for the locomotive itself.^[1]

They were initially numbered 1000-1004 in the steam locomotive number series, but were subsequently renumbered D301-D305 in order. The locomotives were used on yard pilot and transfer freight duties, although number 1000 hauled a freight train from Dublin to Cork during trials.^[1] Two locomotives were stored from 1960 and the rest had followed by 1972, though officially they remained in stock until 1976. All five were scrapped in 1977.

The D Class is not available in either RTR or kit form. However, the British Rail Class 08 can be used as a close approximation.^[2]