The Ulster, D&D and D&BJct railways together formed the main line between Dublin and Belfast, with the D&BJct completing the final section in 1852 to join the Ulster at Portadown.
The GNRI's other main lines were between Derry and Dundalk and between Omagh and Portadown.
The Portadown, Dungannon and Omagh Junction Railway together with the Londonderry and Enniskillen Railway enabled GNRI trains between Derry and Belfast to compete with the Belfast and Northern Counties Railway, and both this and the Dundalk route gave connections between Derry and Dublin.
These main lines supported the development of an extensive branch network serving the southwest half of Ulster and northern counties of Leinster.
The GNRI became Ireland's most prosperous railway company and second largest railway network.

 
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.
Later the company adopted its famous pale blue livery for locomotives (from 1932), with the frames and running gear picked out in scarlet.
Passenger vehicles were painted brown, instead of varnished.
On 12 June 1889, a significant rail accident occurred when a passenger train stalled between Armagh and Newry.
The train was divided, but during the uncoupling operation ten carriages ran away and collided with another passenger train.
A total of 80 people were killed and 260 were injured in what was then the deadliest railway accident to have occurred in Europe.
The accident remains the deadliest ever to have occurred on the island of Ireland.
The Ulster, D&D and D&BJct railways together formed the main line between Dublin and Belfast, with the D&BJct completing the final section in 1852 to join the Ulster at Portadown. The GNRI's other main lines were between Derry and Dundalk and between Omagh and Portadown. The Portadown, Dungannon and Omagh Junction Railway together with the Londonderry and Enniskillen Railway enabled GNRI trains between Derry and Belfast to compete with the Belfast and Northern Counties Railway, and both this and the Dundalk route gave connections between Derry and Dublin. These main lines supported the development of an extensive branch network serving the southwest half of Ulster and northern counties of Leinster. The GNRI became Ireland's most prosperous railway company and second largest railway network.
 
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. Later the company adopted its famous pale blue livery for locomotives (from 1932), with the frames and running gear picked out in scarlet. Passenger vehicles were painted brown, instead of varnished. On 12 June 1889, a significant rail accident occurred when a passenger train stalled between Armagh and Newry. The train was divided, but during the uncoupling operation ten carriages ran away and collided with another passenger train. A total of 80 people were killed and 260 were injured in what was then the deadliest railway accident to have occurred in Europe. The accident remains the deadliest ever to have occurred on the island of Ireland.
Great Northern Railway (Ireland)