What 'new rules'? New rules in your head more like. :D
Would or are?
If they are under 21 and haven't played for the 'A' team. Personally I'd like to see a return to the days that apart from schools representative sides, you should not play for more than one country.
Odd that someone like me with a 'ancestral link' to what you term 'the south (sic.)' and with Irish citizenship can't pass on my citizenship to my children without a FBC, while those born in that equally foreign, and in no way connected to the 26C, part of the world you call 'Northern Ireland', can. My wife has an ancestral link (mother) to the 26C, and lived in the 26C for more than two years, but can't pass on her Irish citizenship automatically; her father who has no ancestral link and never lived in the 26C could because he's from Belfast. What's your point here?
Foreign Births Registration
A person born abroad to a parent who, although not born in Ireland, was otherwise an Irish citizen at the time of the person’s birth, can become an Irish citizen by applying for Foreign Births Registration, either to the Irish Diplomatic or Consular Mission nearest to where the applicant normally resides...
see http://www.dfa.ie/home/index.aspx?id=267
To answer your question, maybe they can. All they need is a birth certificate showing that they were born in Ireland prior to 2004 to prove they're entitled to Irish citizenship.