[2] Construction began in 1903; the first passenger services did not run until 31 May 1909, when the 10-mile (16 km) between Armagh and Keady was opened,[3] although goods trains had started in March 1908.

[4] The 8-mile (13 km) section between Castleblayney and Keady was opened on 11 November 1910,[5] and the GNR (which was working the line) took over the company on 1 June 1911.

[7] The Partition of Ireland in 1922 turned the Armagh–Monaghan county boundary between Creaghanroe and Carnagh into an international frontier that resulted in the GNR closing the Castleblaney – Keady section from 2 April 1923 (the customs border came into effect on Sunday 1 April) [8] and withdrew passenger services from the Keady – Armagh section from 1 February 1932.

The line's summit at Carnagh was 613 feet (187 m) above sea level, the highest place on the GNR.

[11] This is a substantial saving in weight and construction compared with earlier purely stone or brick viaducts.