"Prior to the Good Friday Agreement, the Constitution of Eire (the 26 Counties) claimed dominion over the 6 Counties of Northern Ireland. That meant, those of us born in NI were offered dual citizenship should we wish to take it up."
The GFA didn't change anything - anyone who has a grandparent born in either the Republic or N Ireland can still apply for a Republic birth certificate and once you have that an Irish Passport.
the paperwork is pretty heavy as you have to get copies of births deaths and marriages certificates to prove line of descent. Pre Covid it took about 6-8 months to get the Birth Cert and then about 3 months to get the passport.
Everything was b put on hold 15 months ago so there is an enormous backlog waiting when they do restart
So if you are thinking of it start right now. A colleague of mine always said you can't have too many passports in these troubled times.
Note I don't think that this allows you to pass on citizenship or get a passport of the Republic for your children or spouse if they themselves have no connection to the Republic or NI