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 Dundalk at the GNR Works the railway engineers developed railbuses for use on sections of the rural network.
Nevertheless, by the 1950s the GNRI had ceased to be profitable and in 1953 the company was jointly nationalised by the governments of Ireland and Northern Ireland.
The two governments ran the railway jointly under a Great Northern Railway Board until 1958.

In May 1958 the Government of Northern Ireland's wish to close many lines led to the GNRI Board being dissolved and the assets divided between the two territories.
At midnight on 30 September 1958 all lines in Northern Ireland were transferred to the (nationalised) Ulster Transport Authority (UTA) and all lines in the Republic of Ireland were transferred to Córas Iompair Éireann (CIÉ).
CIÉ had been formed as a private company in 1945 but had been nationalised in 1950.
In an attempt at fairness, all classes of locomotive and rolling stock were also divided equally between the transport operators of the two new owners.
Most classes of GNRI locomotive had been built in small classes, so this division left both railways with an operational and maintenance difficulty of many different designs all in small numbers.

The Northern Ireland Government, which had a very anti-rail policy, rapidly closed most of the GNRI lines in Northern Ireland.
Exceptions were the Belfast–Dundalk and Portadown–Derry main lines and the Newry–Warrenpoint and Lisburn–Antrim branches.
It made the Lisburn–Antrim branch freight-only from 1960 and closed the Portadown–Derry and Newry–Warrenpoint lines to all traffic in 1965.
The Republic of Ireland government tried briefly to maintain services on lines closed at the border by the Northern Ireland government, but this was impractical, and the Republic had to follow suit in closing most GNRI lines south of the border.
Since 1960 the Drogheda–Navan branch has survived for freight traffic only.

The GNR's north western main line between Dundalk and Derry bypassed the small County Tyrone town of Fintona, which was instead served by a 1 mile (1.6 km) branch line from Fintona Junction station.
The service was operated by the double-deck Fintona horse tram until the line's closure in 1957.
CIÉ also acquired the Hill of Howth Tramway, in the northern suburbs of Dublin, in the 1958 dissolution of the GNRI Board.
CIÉ closed the tramway about a year later.

Today, the GNR routes remaining consist of the main line from Dublin to Belfast, the Howth branch, electrified for Dublin commuter services since 1984, the Drogheda - Navan (Tara Mines) line, which carries only freight traffic associated with that mine, passenger traffic having ceased with the closure of the line beyond there to Oldcastle in 1963, and the Lisburn to Antrim branch, now mothballed but retained in operational order for the time being.
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 Dundalk at the GNR Works the railway engineers developed railbuses for use on sections of the rural network. Nevertheless, by the 1950s the GNRI had ceased to be profitable and in 1953 the company was jointly nationalised by the governments of Ireland and Northern Ireland. The two governments ran the railway jointly under a Great Northern Railway Board until 1958.
In May 1958 the Government of Northern Ireland's wish to close many lines led to the GNRI Board being dissolved and the assets divided between the two territories. At midnight on 30 September 1958 all lines in Northern Ireland were transferred to the (nationalised) Ulster Transport Authority (UTA) and all lines in the Republic of Ireland were transferred to Córas Iompair Éireann (CIÉ). CIÉ had been formed as a private company in 1945 but had been nationalised in 1950. In an attempt at fairness, all classes of locomotive and rolling stock were also divided equally between the transport operators of the two new owners. Most classes of GNRI locomotive had been built in small classes, so this division left both railways with an operational and maintenance difficulty of many different designs all in small numbers.
The Northern Ireland Government, which had a very anti-rail policy, rapidly closed most of the GNRI lines in Northern Ireland. Exceptions were the Belfast–Dundalk and Portadown–Derry main lines and the Newry–Warrenpoint and Lisburn–Antrim branches. It made the Lisburn–Antrim branch freight-only from 1960 and closed the Portadown–Derry and Newry–Warrenpoint lines to all traffic in 1965. The Republic of Ireland government tried briefly to maintain services on lines closed at the border by the Northern Ireland government, but this was impractical, and the Republic had to follow suit in closing most GNRI lines south of the border. Since 1960 the Drogheda–Navan branch has survived for freight traffic only.
The GNR's north western main line between Dundalk and Derry bypassed the small County Tyrone town of Fintona, which was instead served by a 1 mile (1.6 km) branch line from Fintona Junction station. The service was operated by the double-deck Fintona horse tram until the line's closure in 1957. CIÉ also acquired the Hill of Howth Tramway, in the northern suburbs of Dublin, in the 1958 dissolution of the GNRI Board. CIÉ closed the tramway about a year later.
Today, the GNR routes remaining consist of the main line from Dublin to Belfast, the Howth branch, electrified for Dublin commuter services since 1984, the Drogheda - Navan (Tara Mines) line, which carries only freight traffic associated with that mine, passenger traffic having ceased with the closure of the line beyond there to Oldcastle in 1963, and the Lisburn to Antrim branch, now mothballed but retained in operational order for the time being.
Great Northern Railway (Ireland)