Played alongside each country's own national competitions, the teams were split into two groups (of 8 and 7) and played a series of round-robin matches with each team playing the other only once. The top four teams from each group proceeded into the knock-out phase until a champion was found. Clashes between teams in the 2001–02 Welsh-Scottish League also counted towards the new competition.
The 2001–02 competition was dominated by the Irish teams with all four sides reaching the last eight, three progressing to the semi-finals, and the final played at Lansdowne Road contested between Leinster and Munster with Leinster running out 24–20 winners. Leinster's 10–0 debut 'perfect season' is one of only two in the history of the competition in its various forms. The other, a 17–0 record, was also achieved by Leinster was in 2020.
In 2001, an agreement was made between the Irish Rugby Football Union (IRFU), Scottish Rugby Union (SRU) and Welsh Rugby Union (WRU) to create a new competition which would bring in the four Irish provinces. 2001 would see the very first incarnation of the Celtic League.
Note: Flags to the left of player names indicate national team as has been defined under IRB eligibility rules, or primary nationality for players who have not yet earned international senior caps. Players may hold one or more non-IRB nationalities.
