It's even observable amongst people from around England who move to London.

They did not believe he was English or upper class. He was just blending in. Plenty of Irish, Australians', New Zealanders' and South Africans' accents change through association when they have lived in Britain for awhile. It's less natural for an expat American's accent to change.

but he TOLD them he was IRISH. he was honest and even knew theyd find it charming. his nickname with matthew goode's character was "IRISH" yet they believed he was a member of upper english class? i get him doing that with other people but he spoke with a british accent from the start with this family all the while telling htem hes from ireland. so no that makes no sense.

Because he's a social climber trying to convincingly pose as a member of the English upper class, which has historically disdained the Irish. It's not complex.