Sean (written "Seán" or "Séan" in Irish) is a Hibernization of the English name "John"; that is, it's a transliteration of "John" into a form which can be pronounced in Irish and written with the Irish alphabet (which nowadays is simply a version of the Roman alphabet).
There were references to the origin of the above idiom, my favorite (supported here as well) being from the movie, The Untouchables, wherein Sean Connery utters with contempt, "Isn't that just like a [racist for Italian]... brings a knife to a gun fight."