After 1945 the GNR(I) found itself in a poor financial position as a result of the deprivations of World War II and increased competition from road traffic. It looked to diesel power as a way to streamline costs and bring them back into profitability. Along with diesel locomotives, railcars offered a cheaper and more flexible means of traction than steam-hauled coaches. As a result, in June 1950 the GNR(I) introduced the first of 20 diesel-mechanical railcars (numbers 600–619) ordered from AEC Ltd. of Southall.
These cars, based on a 1930s AEC/Great Western Railway design, were powered by two AEC underfloor engines of 125 hp, each one driving the inner axle of one bogie through a five-speed, pre-selective epicyclic gearbox. They cost £18,500 each (equivalent to £390,000 in 2016) and were capable of 70 miles per hour (110 km/h).
The bodywork was built by Park Royal Vehicles, which like AEC was part of the ACV Group. The design incorporated a full width cab at the front and a guard's and luggage compartment at the rear, as well as a steam-generating unit. Passenger accommodation was in two saloons, a 12-seater first-class saloon, located immediately behind the driving cab, giving passengers a clear view to the line ahead, and 32 seater third-class saloon.
The power cars could operate with either one or two unpowered intermediate coaches. These coaches were built by the GNR(I) at its Dundalk Works.
The GNR(I) railcars were painted in cream & dark blue
Although limited to four coaches, the AEC's were put in service on the Dublin – Belfast Enterprise service. As a result, these units had the distinction of being the first successful main line diesel railcars in either Ireland or Britain.
Upon dissolution of the GNR in 1958, 10 AEC's went to CIÉ (Where they joined the existing fleet of AEC railcars, with each vehicle number receiving an 'n' suffix), and 10 went to the UTA. The UTA had experimented with AEC railcars in 1951 as well, turning out two power cars, No.'s 6 and 7. The former GNR's AEC's (Now renumbered 111 -120) were technically compatible with 6 and 7.
All 20 were scrapped during the 1970s- CIÉ's inherited 10 were not suitable for conversion to push-pull like their original 60, and were cut up in Mullingar in 1975. The UTA's inherited 10 were subsequently inherited by NIR in 1968 and withdrawn by their new owners in 1972.