The IRFU and the Welsh Rugby Union (WRU) have announced a new development competition for emerging professional players from the two countries.
The Celtic Cup will be open to development squads from Leinster, Munster, Ulster, Connacht, Blues, Dragons, Scarlets and the Ospreys.
Running over seven consecutive weeks beginning on the weekend of September 7-8, the eight teams will be divided into two conferences of four Welsh regions and four Irish provinces.
Each Irish province will play each of the Welsh regions and will play against two Irish provinces.
Similarly, the Welsh regions will play each of the Irish provinces and will play two of their Welsh counterparts.
"The Celtic Cup will bring significant value to the development of our emerging professional players, referees and coaches and support staff," David Nucifora, IRFU Performance Director, said.
WRU Head of Rugby Performance Geraint John added, "The block programme of matches will greatly help all involved to build week to week in a professional environment and competition structure, helping to underpin the development of future regional and international players."
Fixtures for the opening weekend are:
Munster v Connacht, Irish Independent Park KO 17.30
Ulster v Leinster, Malone RFC, KO 17.00
Ospreys v Scarlets, Brigend RFC, KO 19.00
Dragons v Cardiff Blues, Rodney Parade KO 19.15