Derry-born Darron Gibson has opted to represent the Republic

The FAI says it has received a letter from international body Fifa which makes clear Northern Ireland-born players can represent the Republic.

However, the IFA has a different interpretation of the Fifa rules.

Former NI under-age players Tony Kane and Michael O'Connor made Republic under-21 debuts last week.

And Londonderry-born Manchester United player Darron Gibson and Portsmouth's Marc Wilson have been included in several recent Republic of Ireland under-21 squads.

IFA Chief Executive Howard Wells recently raised the issue at a meeting of his organisation's executive committee and then wrote a letter to Fifa "expressing concern and seeking advice and clarification".

The IFA contends that for Northern Ireland-born players to represent the Republic that either a parent or grandparent has to be born in the Republic or the player has to have lived there for at least two years.

However, the FAI's stance on the issue is at variance with the Northern Ireland football governing body.

"On October 20, Fifa's Heinz Tannler, Director of Legal Division, and Corina Luck, Head of General Legal, wrote a joint letter to the IFA, which was copied to the FAI," said a FAI statement published in the Irish News.

"In it, they informed the IFA that 'the existing situation in Northern Ireland allows players to choose whether they wish to represent Northern Ireland or the Republic of Ireland'."

The FAI added that it is scheduled to meet with IFA officials next month when "eligibility will be one of the topics up for discussion".