When the Irish Free State split off from the United Kingdom in the early 1920's, the Irish Rugby Football Union agreed to maintain its' unified status. Therefore all players from the geographic island of Ireland represent a unified Irish team, be they citizens of the Republic of Ireland or the United Kingdom. That's why the IRFU has its' own anthem called "Ireland's Call". They didn't have to go that way -- the Irish Football Association did split, which is why the northern counties field a different squad than the Republic in international soccer.

The Irish rugby team draws on players from both Northern Ireland and Southern Ireland.

They all play for Ireland that's why the irish rugby anthem is "Ireland's Call" (as someone said before). If you've watched Ireland v Wales (yesterday), you have probably seen Rory Best (the hooker) who plays for Ireland and also for the Ulster team. Hope you enjoyed the game :D

Its an all island team.