Ireland's most impressive railway engineering feat is undoubtedly the magnificent Boyne Viaduct in Drogheda. Its opening in 1855 united the railway line between Belfast and Drogheda with that from Dublin to Drogheda (prior to ths, through passengers had to travel through Drogheda town centre to get between the stations on either side of the Boyne). It was designed by one of the premier railway engineers of his time, John Benjamin MacNeill, born at Mountpleasant outside Dundalk. He also designed the elegant Craigmore viaduct and the lovely Egyptian arched-bridge which take the railway line around Newry.

Dundalk was once the most important railway engineering town in Ireland. The engineering works of the Great Northern Railway employed hundreds until their closure in the late 1950s, the network they once served killed off by the disruption caused by Partition (too many stops for Customs inspections) and by competition from road traffic. Trains once ran out of Dundalk in four directions, south to Dublin and north to Belfast, as they still do, but also west, as far as Bundoran on the Atlantic coast of Donegal and north-east to Greenore and on to Carlingford, Newry town centre and points north, not to mention all the possible connections of this rich network. Greenore is still a fascinating relic of the railway age, designed and built in the style of an english industrial town, which served the railway hotel (built 'over' the station) and the port, with its ferries to Holyhead in Wales. Greenore is still an incongrous delight in the Cooley countryside. All over the county, much of the well-built railway infrastruture still survives after some sixty years of redundancy.

All the county's abandoned railway lines and associated infrastructure were surveyed in 2007 (together with those of County Monaghan). This survey is now available online, thanks to Monaghan County Council's IT department.