Rail gauge in Ireland

The track gauge adopted by the mainline railways in Ireland is. This unusual gauge is currently otherwise found only in the Australian states of Victoria, southern New South Wales (as part of the Victorian rail network) and South Australia (where it was introduced by the Irish railway engineer F. W. Sheilds), and in Brazil. Historically, the Grand Duchy of Baden State Railway also used this gauge, between 1840 and 1845 and in the early years of the Canterbury Provincial Railways in New Zealand.

Ireland's first railway, the Dublin and Kingstown, was built to the now standard. The Ulster Railway (UR), taking the Irish Railway Commission's advice used. However, the Dublin and Drogheda Railway began building their new line to on the grounds of lower costs. Following complaints from the UR, the Board of Trade investigated the matter, and in 1843 recommended the use of and that compensation be paid to the UR for the costs incurred in changing to the new gauge.

This was given legal status by the Railway Regulation (Gauge) Act of 1846 which specified 4 feet 8 ½ inches for Great Britain, and 5 feet 3 inches for Ireland.

In 1846, the Ulster Railway was re-gauged at a cost of £19,000 and the Dublin and Kingstown Railway in 1857, at a cost of £38,000.

The Hill of Howth Tramway and the Dublin and Blessington Steam Tramway adopted the gauge. The original Dublin tram system used 5 ft 2-3/16 in (1580-mm) grooved rail which could accommodate railway wagons. However, Dublin's Luas tram system, opened in 2004, uses standard gauge.

Numerous narrow-gauge systems were built, usually to a gauge of : see the list below. Most are now closed, including the largest narrow-gauge system in Ireland or Great Britain: the County Donegal Railways Joint Committee. The Irish narrow gauge today survives as heritage railways in both the Republic and in Northern Ireland. Bord na Móna uses narrow gauge in the Midland's bogs as part of its peat transport network. There is also a private peat railway on the southern shores of Lough Neagh in Northern Ireland, operated by the Sunshine Peat Company.