The infamous Keady Tunnel in Co. Armagh never saw a train run through it, being built under the short lived Keady to Castleblayney railway line (1910-1923).
The reason for its existence was the Ulster and Connaught Light Railway – a madcap scheme to build a narrow gauge railway from Greenore, Co. Louth, to Clifden Co. Galway, via Newry, Bessbrook, Keady, Tynan, Maguiresbridge, Bawnboy Road, Dromod, Rooskey, Tuam and Cong to Clifden.
Some of this network would use existing railways but the most significant section from Dromod to Clifden would be entirely new. As this line was authorised around the same time as the line from Castleblayney to Armagh, the builders of the latter had to accommodate the proposal with a bridge under their line at Keady.
As the U&CLR plans never came to fruition, the tunnel has never had a train run through it.
I have created a webpage in the Gazetteer for Keady Tunnel and this (and other tunnels) will feature in the next map upgrade.