He was born in Ireland which makes him pretty much a native speaker, maybe bilingual with Gaelic. And McCormack is a British name, though Scottish rather than Irish. Mc is Scottish Gaelic for "son of", which in Ireland is O' (as in O'Malley).
You have to keep in mind that there is a wide variety to British pronunciation of English, both regional and social. British English has strong dialects and sociolects which make themselves felt even when the speaker tries to speak standard English. You can usually tell an Irish from a Scottish from a Welsh speaker by the way they pronounce words. If they really let fly they can even be mutually incomprehensible.
Here's a very early version of the sketch "The Four Yorkshiremen": YouTube - Original 4 Yorkshiremen Sketch with Monty Python members and Marty Feldman. They are trying for a bit of a Yorkshire accent (for instance saying "tuff" for "tough"). You can compare that to other versions of the same sketch on Youtube. As an aside, I would say that I learned most of my English from Monty Python and from reading books and singing along badly to music. Definitely did not learn it in school, except for the very basics.
As an approximate rule you can say that there is wider variety in pronunciation between two people living 100 km apart in Great Britain than in that of two people of similar social standing living on the East and West Coast of the US respectively.