The Celtic League is the top-level league competition in Ireland, Scotland and Wales. The first season was contested by 15 sides in 2001-02, with the four Irish provinces being joined by Edinburgh and Glasgow and all nine Welsh Premiership sides. The competition was originally played alongside the Welsh-Scottish League and Irish Inter-Provincial Championship, a trend that only lasted for a single season with the Welsh-Scottish league abandoned and Irish Championship downgraded.

The league was originally contested over two groups, with semi-finals and a final deciding the winners.

In 2004, the league became the major domestic competition in all three countries, with the five Welsh regions (Cardiff Blues, Ospreys, Scarlets, Dragons and the now defunct Celtic Warriors) taking the place of sides from the Premiership. The league also embraced a traditional league structure for the first time.

Celtic Warriors were disbanded at the end of the 2004-05 season, and Scottish territory Border Reivers suffered the same fate at the end of the 2006-07 season. A semi-final and final play-off system will be re-introduced for the 2009-10 season.