The 1998–99 Irish League Cup was the 13th edition of Northern Ireland's secondary football knock-out league cup competition. It concluded on 4 May 1999 with the final.
Linfield were the defending champions after their fourth League Cup win last season; a 1–0 victory over Glentoran in the previous final. This season they became the first club ever to successfully defend the trophy, and it was a repeat of the previous final, Linfield once again coming out on top with a 2–1 victory against Glentoran in the final, lifting the cup for the fifth time and condemning the Glens to defeat in the League Cup final for the third season running, which is a record for successive final defeats in the competition that still stands.
The competition was re-structured this season, allowing only the 10 top-flight clubs, and the 8 second-tier clubs to enter. This reduced the number of clubs taking part from the usual 32, down to 18.