2 removed above & pointed to the specific @handle. removed uncommon accronyms
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The above answer by @posdef is incorrect: each. Each sport can set their own rules. I worked for the Welsh Athletics federation and there were some very stringent rules (birth; ancestry;birth, ancestry, residency) over who could and could not compete for Wales in international competitions (notwithstanding the fact that British athletes compete as GBGreat Britain & NINorthern Ireland in international champs like Europeans and Worlds) and the Commonwealth Games. We just couldn't pick any British athletes who were interested (it used to be much looser but it was tightened as it provoked resentment).

I am not sure about other sports.

As for the Northern Ireland/Rep of Ireland debate, all people born in NINorthern Ireland are granted dual citizenship so are eligible for both (as in Rory McIlroy's case recently) - assuming the sport hasn't got a unified Irish team like as Rugby.

The above answer is incorrect: each sport can set their own rules. I worked for the Welsh Athletics federation and there were some very stringent rules (birth; ancestry; residency) over who could and could not compete for Wales in international competitions (notwithstanding the fact that British athletes compete as GB & NI in international champs like Europeans and Worlds) and the Commonwealth Games. We just couldn't pick any British athletes who were interested (it used to be much looser but it was tightened as it provoked resentment).

I am not sure about other sports.

As for the Northern Ireland/Rep of Ireland debate, all people born in NI are granted dual citizenship so are eligible for both (as in Rory McIlroy's case recently) - assuming the sport hasn't got a unified Irish team like as Rugby.

The answer by @posdef is incorrect. Each sport can set their own rules. I worked for the Welsh Athletics federation and there were some very stringent rules (birth, ancestry, residency) over who could and could not compete for Wales in international competitions (notwithstanding the fact that British athletes compete as Great Britain & Northern Ireland in international champs like Europeans and Worlds) and the Commonwealth Games. We just couldn't pick any British athletes who were interested (it used to be much looser but it was tightened as it provoked resentment).

I am not sure about other sports.

As for the Northern Ireland/Rep of Ireland debate, all people born in Northern Ireland are granted dual citizenship so are eligible for both (as in Rory McIlroy's case recently) - assuming the sport hasn't got a unified Irish team like as Rugby.

1
source | link

The above answer is incorrect: each sport can set their own rules. I worked for the Welsh Athletics federation and there were some very stringent rules (birth; ancestry; residency) over who could and could not compete for Wales in international competitions (notwithstanding the fact that British athletes compete as GB & NI in international champs like Europeans and Worlds) and the Commonwealth Games. We just couldn't pick any British athletes who were interested (it used to be much looser but it was tightened as it provoked resentment).

I am not sure about other sports.

As for the Northern Ireland/Rep of Ireland debate, all people born in NI are granted dual citizenship so are eligible for both (as in Rory McIlroy's case recently) - assuming the sport hasn't got a unified Irish team like as Rugby.