Ooh, I've got a great angle on it...
Read the "Conan" series by Robert Howard (THE American fiction god)... No really, there are some 30 books in the series and they all rock.
R.E.Howard was big into the racial stereotyping thing (and not in a bad way) and his Black-haired Blue-eyed creation Conan was an Atlantean!!!
That's right folks, the Black-Irish are the last of the Atlanteans.
Or a sub-genus evolved from prehistoric pre-celtic hunter-gatherer stock (Straight from Sumeria - all Jet black hair and proto-language) that got trapped when the Euro-Irish land-bridge drowned (ATLANTIS!). I mean - blue eyedness is just an adaptation to cope with all the rain, it wouldn't take more than 500 years.
Am I right?