The 2025-2026 NIR1 season consists of 2 stages. In the first stage, each NIR1 team plays the other 11 NIR1 teams 3 times for a total of 33 league matches. In each 3 match series one team hosts 2 home matches and the other team hosts 1 home match. In the second stage, the top six teams (Championship Group) play each other a 4th time (5 more matches) to determine final rankings for UEFA Champions League and Europa League placements, and the bottom six teams (Relegation Group) play each other a 4th time (5 more matches) to determine the relegation placements. Total points are for all 38 matches. Table ranking ties are broken by (1) goal difference, (2) goals scored, (3) head-to-head points, (4) head-to-head goal difference, then (5) a drawing.

At the end of the 2025-2026 NIR1 season, the 3rd, 4th, 5th and 6th place NIR1 Championship Group teams and the 1st place NIR1 Relegation Group team enter into the UEFAECL playoffs (knockout format). The winner of this playoff receives a berth in the 2026-2027 UEFA Europa Conference League - 1st Qualifying Round.

At the end of the 2025-2026 NIR1 season, the 6th place NIR1 Relegation Group team will be relegated to the NIR2, and the 5th place NIR1 Relegation Group team enters into a Home-Away highest aggregate score playoff with the 2nd place NIR2 team. The playoff winner and the 1st place NIR2 team play in the 2026-2027 NIR1, and the playoff loser plays in the 2026-2027 NIR2.