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Fake Irish - Alan Rickman, Ian Hart and Julia Roberts. Aidan Quinn also had to Irish it up a bit due to having a far more Americanised accent.
Justified, in Rickman's case- the director felt that, so deeply ingrained is the popular image of Éamon de Valera in the Irish national consciousness, it was necessary that he be played by a non-Irishman who lacked such preconceptions and would be able to play the role as a three-dimensional character, rather than a two-dimensional caricature. Dev was also so ethnically diverse (a Spanish Cuban father, born in New York City) that his Irishness isn't really important from a casting point of view.
Rickman also bears an astonishing likeness to de Valera, which helps.