The main lines between Dublin and Cork, Belfast, and Belfast - Derry received a fair amount of investment over the years, and locomotives, carriages and services were as good as anywhere in Europe, but rural and western lines received little new investment especially during the lean years of the 1920's - 1930's.  In 1925, all the railway companies whose lines fell wholly on the Southern side of the newly created border (between Northern Ireland and the Republic) were amalgamated as the Great Southern Railways. By the late 1940's, Ireland still had an extensive railway system which was almost entirely steam operated, and indeed, one short branch line in Co. Tyrone was to remain horse worked until its closure in 1957! 

Between 1949 and 1960, huge changes took place.  Large investment was made in diesel locomotives and railcars and new carriages in the Republic, with Inchicore working at full capacity for many years.  Following nationalisation of CIE (the State transport company in the Republic) in 1950, many unremunerative lines were closed, and others reduced to freight only status.  Meanwhile, in the North, the Ulster Transport Authority (state owned railway and general public transport in Northern Ireland 1949 - 1967)  closed almost 80% of the railway system under its control, and introduced diesel railcars to the rest. 

Both economic circumstances and political interference had resulted in the closure of most of the Great Northern Railway system in 1957.  The remainder was divided between the two state companies.  CIE took over the remaining lines in the Republic, and the UTA took over what was left in the North - only to close over half of it by 1965. 

By 1960, main line trains were almost entirely dieselised, leaving steam on secondary duties and freight traffic.  At the end of 1962, following the delivery of more diesel locomotives and with further branch line closures imminent,  CIE eliminated steam traction for good.  In the North, dieselisation had followed a different pattern.  No main line diesel locomotives had been acquired at all, and practically all passenger trains were railcar formations.  Freight traffic had been abolished by the UTA in 1965, apart from cross border traffic to and from Belfast and Derry, and yet further closures of lines had taken place.  In 1967, the UTA was split up into road and rail undertakings.  The railways became the Northern Ireland Railways system.  Steam lingered on in the form of a remaining few ex-NCC "WT" class 2.6.4T's until 1970, when the last two were withdrawn.  The last public main line passenger train both in Ireland and in the British Isles had operated on Easter Monday, 1970.  After that, NIR began a gradual program of renewal.  Currently, most NIR services are still railcar operated (by newer railcars!) while most services in the Republic are locomotive hauled.