I'm sorry you have had no answer so far. I'm really not entirely sure of the correct answer.
Seven Romans would be septem Romani.
Seven Carthaginians would be septem Carthaginienses.
Since London is (ususally) Londinium, I suppose that seven Londoners would be septem Londinienses.
New York is Novum Eboracum, and I therefore suppose that seven New-Yorkers might be septem Novum-Eboracienses-- I don't see how the Novum can reasonably be declined.
Dublin has been variously rendered in Latin: Dublana, Dublinum, Dublinia, Dublinium, or Eblana. If we pick Dublin(i)um, then seven Dubliners might well be septem Dublinienses.