The British, US and European standard railway gauge is 4'8½". The island of Ireland has a gauge of 5'3". Why this is so is subject to many different versions of events. The generally accepted version is as follows.
The first railway opened in Ireland was the Dublin & Kingstown Railway in 1834. This was built to what became the British standard gauge of 4'8½". The Ulster Railway from Belfast to Portadown was built to a gauge of 6'2". The Dublin & Drogheda Railway was engineered to be able to take a 6'2" gauge, but their engineer, Sir John MacNeill favoured a 5'2" gauge.
Realising the chaos and lack of interoperabilty that would arise if this situation was allowed to continue unchecked, the Board of Trade appointed one of their officers, Major General Pasley to recommend on a suitable gauge for the island of Ireland (then united as part of the United Kingdom). He sought advice from railway engineers as to the most appropriate gauge for railway development.
Leaving aside the Stephenson's choice of 4'8½" and Brunel's 7'0¼" as partial advice, all other answers ranged between 5'0" and 5'6". Acting impartially, he split the difference and arrived at 5'3". This gauge was confirmed as the gauge for all future railway development on the island of Ireland in the Gauge of Railways Act of 1846, which also set the standard gauge for Britain at 4'8½".
This would have been the end of the matter, except that the Act was not retrospective in its application, hence the D&KR and Ulster Railway were not obliged to convert their lines. The Ulster Railway were, however, placed in some difficulty as when renewing powers for their extension to Armagh in 1845, Parliament insisted that this be to the gauge of 5'3". They did, however, agree to levy part of the cost of converting the existing line on the other companies connecting with the UR, as it was rightly argued that these companies would benefit from the conversion.
