Built at Litchurch Lane, Derby, in 1970, eight examples were constructed for Northern Ireland Railways for its new Enterprise service between Belfast and Dublin.
This landmark service featured a range of bespoke vehicles that were quite unlike anything then in operation in Ireland—or indeed on British Rail—despite being a derivative of a BR design.

The eight Mk.2b coaches acquired by the newly formed Northern Ireland Railways (NIR) included several new types: one First Open (FO), two Brake Driving Trailer Seconds (BDTS), and a single Grill-Bar—the latter being the only purpose-built Mk.2 catering vehicle. The other four vehicles were designated SOs, although they were almost indistinguishable from BR TSOs.

Delivered in NIR’s blue and maroon scheme and fitted with B4 bogies set to the Irish 5ft 3in track gauge, they initially worked in push-pull formation on the Belfast–Dublin Enterprise express service, powered by one of the three Hunslet DL Class 101 locomotives—or by pairs of them, either top-and-tailed or double-headed during the summer months.In 1978, the fleet—which now included five Mk.2c coaches purchased in 1972—was refurbished and repainted in a new silver-grey and blue livery.

However, the increasingly unreliable Hunslets—despite sharing an engine and many mechanical and electrical components with the BR Class 20s—were replaced in 1980 with General Motors 111 Class locomotives.The lack of electric train supply on the new motive power required the addition of a generator coach to each Enterprise formation.

In 1980, NIR acquired five ex-BR Mk.2b BFKs for conversion, along with an additional ten secondhand Mk.2b coaches and three Mk.2cs. The use of driving trailers on the Enterprise ended with the introduction of the 111s, and the trains reverted to being hauled as conventional coaching stock.With this extra stock, it became increasingly common to see Enterprise trains away from their usual cross-border route.

Formations of three to five coaches were regularly used on Portadown/Lisburn–Belfast–Bangor and Belfast–Derry/Londonderry services, typically with a single Hunslet operating in push-pull mode. By the mid-1980s, several ex-BR coaches were rebuilt as open coaches, while others were converted to strengthen 80 Class DEMU sets or replace bomb-damaged vehicles.