Why is the Irish railway gauge 5 feet 3 inches?
The original Ulster Railway (Belfast to Armagh) gauge was 6 ft 2 in (1,880 mm) and the Dublin and Drogheda Railway companies used 5 ft 2 in (1,575 mm), both different from the gauge in Great Britain at 4 ft 8½ in (1,435mm).
When connecting lines between the two Irish companies was mooted a standard gauge became necessary.
In 1843 the Board of Trade (with the advice of engineers Charles Pasley and George Stephenson) introduced the gauge of 5 ft 3 in (1,600mm) as a compromise.
The Railway Regulation (Gauge) Act 1846 was passed to formalise this gauge on the island of Ireland.
(References courtesy of Wikipedia)