The United Kingdom consists of Great Britain and Northern Ireland, and previously consisted of Great Britain and the whole of Ireland. Rail transport systems developed independently on the two islands of Great Britain and Ireland, and most of the railway construction in the Republic of Ireland was undertaken before independence in 1922. Thus, the logical division to discuss the history and present-day state of railways in these areas is by geographical division, rather than the political division of nation states.
Similarly, for the history of rail transport, rather than the current situation (described in the above articles), see History of rail transport in Great Britain and History of rail transport in Ireland.