I had meant to respond earlier but I've been distracted by local issues. Railways in Ireland are a complex subject, much more than it might seem superficially.
The two gauges are, as indicated, 5'3" for most railways and 3'0" for a number of secondary lines, many on the Atlantic Coast and in County Donegal in the north west. The most northerly parts of Ireland are on the West Coast, and belong to the Republic, despite the usual name for the British Counties being "Northern Ireland". The County Donegal railways were centred on Derry (or "Londonderry" if you were from the Northern Counties.)
A good book on the locomotives is "Locomotive Compendium Ireland" by Colin Boocock, published by Ian Allan in 2009. It lists the locomotives in service from 1949 until 2009. There have been no new locomotives introduced in Ireland for some years, so it is up to date.
The railways in the Republic were nationalised in 1924 as the "Great Southern Railway" although railways which crossed the new border between the Republic and the Northern Counties remained privately operated.. The Great Northern of Ireland remained private until 1958 when it was divided at the border, with the locomotive fleet being split.
The railways in the Northern Counties remained private until 1948, when they were nationalised by the British Government, forming the "Ulster Transport Authority".
One of the small cross border railways, the Dundalk, Newry and Greenore had been owned by the London and North Western, passed to the LMS in 1923 and to British Railways in 1948. The six locomotives, 0-6-0 Saddle Tanks dating back to 1873, remained in service until the line closed in 1951.
The Northern Counties Committee had been owned by the Midland, then the LMS but passed to the UTA and later Northern Ireland Railways. This company had many locomotives generaly similar to LMS locomotives, but only two were ex LMS standard gauge, two Class 3F 0-6-0 tanks which were regauged in 1944, presumably to assist with traffic during wartime (although Ireland was neutral).
There are many more detailed books, but you really have to want to know that much more about Irish Railways and Locomotives.
Peter
The 5'3" gauge is also used in Victoria and South Australia and in parts of Brazil.
I remember the Irish broad gauge as 5'3", and I seem to remember that this was chosen by averaging some then-common British gauges.
IIRC most of the rail lines in Ireland were built to 5' "broad gauge". In Ulster, one of the old British railways built lines there (based on the links above, apparently it was LMS), and those were standard gauge. I believe there were also narrow gauge railways that served the peat business.
THANKS ! Peter [M636C], for sharing your Thread here about The R.P.S.I., and their really interesting website. (See linked in OP's post).... As someone who has not traveled to either country; (Britan, or Ireland); I've seen/ or read, but little exposure to the rails in G.B. But Ireland is still pretty merky(?) for many of us, of limited world travel experience...[My only overseas experiences were in SE Asia...The natives were still shooting at us, in those times]
Ireland looks pretty interesting, and not having seen it, first hand, some of the abreviations used were somewhat 'odd' , and how many 'gauges' exist there? Are all the 'railroads', in Ireland owned or operated by their "State' governments(?).. Thanks .
The following photograph had been misidentified in another thread.
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