These main lines supported the development of an extensive branch network serving the southwest half of Ulster and northern counties of Leinster.

In its early years the GNR(I) closely imitated the image of its English namesake, adopting an apple green livery for its steam locomotives and a varnished teak finish for its passenger coaches.

The Great Southern & Western Railway had introduced express passenger locomotives with a 4-6-0 wheel arrangement, and the GNRI wanted to do the same.

However, the lifting shop in the GNRI Dundalk works was too short to build or overhaul a 4-6-0, so the company persisted with 4-4-0 locomotives for even the heaviest and fastest passenger trains.

The imposition of border controls caused some service disruption, with main line trains having to stop at both Dundalk and Goraghwood stations.

A combination of the increasing road competition facing all railways and a change in patterns of economic activity caused by the Partition of Ireland reduced the GNRI's prosperity.

The company modernised and reduced its costs by introducing modern diesel multiple units on an increasing number of services in the 1940s and 1950s and by making Dublin–Belfast expresses non-stop from 1948.

In May 1958, the Government of Northern Ireland's wish to close many lines led to the GNR(I) Board being dissolved and the assets divided between the two territories.

The RPSI periodically operates one or more of them on special excursion trains on Northern Ireland Railways and Iarnród Éireann (successor to CIÉ) routes.

1938 built dining car No.88 still sees use as part of the Railway Preservation Society of Ireland's Dublin-based "heritage set" of coaches.

Other now disused stations are of architectural interest as are the early 20th century concrete structures at the 1910 Tassagh Viaduct and the 1926 Clones Engine House.