Kammala jumla mai zuwa: Farawa tare da ƙirƙirar Welsh <unk> Scottish League a 1999, an san gasar ne a matsayin Celtic League lokacin da ya girma ya hada da kungiyoyi daga Ireland, Scotland da Wales. Kungiyar ta tallafawa kamfanin kera cider na Irish Magners daga kakar 2006-07 har zuwa 2010-11. A farkon kakar 2010/11, an fadada gasar daga kungiyoyi 10 zuwa 12, tare da kara kungiyoyin Italiya biyu. Bayan ƙarshen tallafin Magners, RaboDirect ya tallafawa gasar daga
2011<unk>12 zuwa 2013<unk>14. An karɓi sunan Pro12 a cikin 2011 don nuna cewa gasar yanzu ta haɗa da ƙungiyoyi daga wajen ƙasashen Celtic. Ƙarin faɗaɗa zuwa ƙungiyoyi 14 (da sake sake suna zuwa Pro14) ya faru daga kakar 2017-18, tare da ƙungiyoyin Afirka ta Kudu biyu da ke shiga Championship har zuwa kakar 2019-20. Yarjejeniyar tallafawa ta kwanan nan tare da Guinness ta fara ne a farkon kakar 2014-15, kuma an kammala bayan Pro14 Rainbow Cup a watan Yulin 2021. Bayan sake fasalin zuwa United Rugby Championship, gasar ta kafa sabon haɗin gwiwa tare da Roc Nation.