The Irish Football Association Challenge Cup, commonly referred to as the Irish Cup (currently known as the Clearer Water Irish Cup for sponsorship purposes) is the primary football knockout cup competition in Northern Ireland. Inaugurated in 1881, it is the fourth-oldest national cup competition in the world. Prior to the break-away from the Irish Football Association by clubs from what would become the Irish Free State in 1921, the Irish Cup was the national cup competition for the whole of Ireland.

Since 3 October 2023, the cup has been sponsored by Clearer Water. It was previously sponsored by Nationwide Building Society, Bass Ireland Ltd, JJB Sports, Tennent's Lager and Sadler's Peaky Blinder and Samuel Gelston's Irish Whiskey.

Dungannon Swifts are the current holders after defeating Cliftonville 4–3 on penalties in the 2025 final.

During the cup's history, different formats and rules have been used in respect of eligibility to enter the competition, the number of teams and rounds, replays, extra time, penalties, etc. The competition is open to all IFA-affiliated clubs with intermediate or senior status. Clubs obtain such status by meeting minimum criteria laid down by the IFA in respect of facilities, etc. Each club, for example, must have its own enclosed ground. The competition usually begins in August or September with the first qualifying round, and ends with the final in May of the following year. Clubs from level 3 (the NIFL Premier Intermediate League) and all regional league entrants from level 4 and below enter in the first qualifying round. These clubs play against each other over four qualifying rounds, until 8 clubs remain. The 8 fourth qualifying round winners then join the 24 senior clubs from levels 1 and 2 of the Northern Ireland Football League in the first round of the main competition, which consists of 32 clubs and is played in January.

The competition has always been played in a randomly-drawn knockout format. Depending on the number of entrants, the draw sometimes necessitates byes in the earlier rounds. If a team receives a bye, they automatically get drawn first in the following round to avoid the possibility of a team receiving multiple byes. The team drawn first from each tie hosts the match, except in the case of an intermediate team from level 3 or below being drawn first against a senior team from levels 1 or 2. In that scenario, the tie is played at the ground of the 'senior' team. In all ties that finish level after 90 minutes, extra time is played and (if necessary) penalties are used to decide the winner. Both semi-finals and the final are usually played at Windsor Park.

The winners qualify to represent Northern Ireland in the following season's UEFA Europa Conference League, subject to the club attaining a UEFA licence. However, if they have already qualified for a UEFA competition by finishing as champions or runners-up of the NIFL Premiership, or if they do not attain a UEFA licence, the Irish Cup's Europa Conference League berth is redistributed to the third-placed Premiership club, and the 4th–7th placed clubs participate in the Premiership's Europa Conference League play-offs.

The Irish Cup was inaugurated in the 1880–81 season, with seven clubs taking part. The draw for the first round took place on 10 January 1881, with the first ever Irish Cup matches being played on 5 February 1881. The trophy was first awarded to Moyola Park (from Castledawson in County Londonderry) when they beat Cliftonville (from Belfast) 1–0 in the final at Cliftonville Cricket Ground, Belfast on 9 April 1881. Since its inception, the Irish Cup has always been, and continues to be, considered the most important such competition in Northern Ireland (and, prior to 1921, Ireland), second only to the NIFL Premiership. The cup final is the climax of the domestic season in Northern Ireland and usually attracts the biggest attendance of any club match.

Since the inception of the Irish Football League in 1890–91 (and excluding the First and Second World War years when the League was suspended), the Cup has been won by Irish League clubs on every occasion except three famous ‘giant-killing’ occasions when "junior" clubs beat senior opponents in the final: in 1928, Willowfield beat Larne 1–0; in 1955, Dundela beat Glenavon 3–0; and in 1976, Carrick Rangers beat Linfield 2–1. In the early years, Army regiments stationed in Ireland entered teams such as King's Own Rifles (Cork), three of which reached the final: the Gordon Highlanders in 1890, the Black Watch (Limerick) in 1892 and the Sherwood Foresters (Curragh, County Kildare) in 1897. The Gordon Highlanders were the only Army team to win the Cup.