Following the introduction of regional rugby union teams in Wales ahead of the 2003/04 season, the format of the Celtic League tournament was changed so that each team would play each other twice, home and away, with the side that accumulated the most points during the season winning the title. This was a change from previous seasons, which culminated in a knock-out format competition leading to a final. In a bid to attract broadcasters, sponsors and the public by having a 'showpiece' final, the Irish, Scottish and Welsh unions agreed to launch a new cup competition, with a knock-out format, the Celtic Cup.
2003–04 season
During the inaugural season, the competition was contested by all 12 sides that contested the Celtic League and ran concurrently with the league on four weekends between September and December 2003. The tournament was a knock-out format, played over one leg with the first team drawn in each fixture hosting the match. Due to the number of teams competing, eight teams contested the first round in mid September whilst four teams, Edinburgh, Munster, Ulster and Blues were given byes to the quarter finals to meet the victors from the first round. The victorious teams from the first round were Connacht, Glasgow, Leinster and the Llanelli Scarlets. The quarter-finals took place on the first weekend of October, with Edinburgh, Glasgow and Connacht winning their games. Ulster and Leinster drew their game 23–23 after extra time, however Ulster progressed under the competition rules as they scored three tires in the match against Leinster's two. The semi-finals took place in mid November and saw both the away teams win, as Ulster beat Glasgow and Edinburgh beat Connacht. The final was held on Saturday December 20 at Murrayfield Stadium and saw Ulster beat Edinburgh 21–27 to win the Celtic Cup.
2004–05 season
The format of the competition was changed for the second edition of the competition. The competition was moved to April and May to run after the conclusion of the Celtic League competition, with only eight sides contesting the competition, again in a straight knockout format, however starting at the quarter-final stage due to the reduction in teams. The quarter-final fixtures were created based on the teams' finishing positions in the Celtic League, with the league winners Neath-Swansea Ospreys hosting the eighth-placed side Ulster, second-placed Munster hosting seventh-placed Edinburgh, third-placed Leinster hosting sixth-placed Glasgow and fourth-placed Newport Gwent Dragons hosting fifth-placed Llanelli Scarlets. In this way, the tournament was similar to a play-off system, although the Celtic League and Celtic Cup remained trophies in their own right. The automatic home advantage for the highest-placed teams was not continued for the semi-finals, instead the fixtures were decided by a draw. Leinster lost to Munster and Scarlets won at home against the Ospreys. The final took place on 14 May at Lansdowne Road and saw Munster beat the Scarlets by 27 points to 16 to win the second Celtic Cup.
Demise
Following the end of the 2004–05 season, the Welsh regions signed a deal to join the English Premiership club sides in an Anglo-Welsh Cup competition from the 2005–06 season. With clashing fixtures in the two competitions, the fallout led to the Irish and Scottish sides withdrawing from Celtic competition at the end of May. Talks between the three countries in June led to an agreement to reinstate the Celtic League, but not the Celtic Cup. For the 2009–10 season, the Celtic League adopted a play-off format similar to that used for the 2004/05 Celtic Cup, with the top four placed teams in the league at the end of the season rather than eight. However the Cup moniker was not revived and the winners of the play-offs were instead crowned the overall Celtic League champions.
