Translation:It is between the American and the Irish person.
There are a couple of reasons why that's not a valid answer.
The first is that it is a copular statement in Irish - you say Is Meiriceánach mé, not Tá mé Méiriceánach. While Méiriceánach can be both a noun and an adjective, and "I'm American" and "I'm an American" are both valid in English, Tá mé Meiriceánach is not used in Irish.
The second problem is that idir can mean either "between" or both, but the "both" meaning causes lenition.
Is idir Mheiricéánach agus Éireannach é.