International rugby union eligibility rules
Regulations relating to the eligibility of players to play for national teams in rugby union, both in the fifteen-a-side game and rugby sevens, are the responsibility of World Rugby, the governing body for the sport. Players have been able to play for a national team based on birth, a period of residency, or having a parent or grandparent born in that country.
Source: Wikipedia — International rugby union eligibility rules (CC BY-SA 4.0)