The Newry & Armagh Railway (originally planned to be the Newry & Enniskillen Railway) built a line between the two locations in its name. This included the 1,759 yard long Lissummon Tunnel just west of Newry and the Dublin – Belfast Railway.
The tunnel is now on private property and access not officially permitted (I have never been in, but if the owner sees this post, I’d be happy to pay a visit with permission).
Youtuber Stephen Reid has a video on his channel of a recent visit to Lissummon tunnel.