Ties in with the stereotype that Irish men are not very articulate or compassionate. If something was particularly unpleasant or an anti-climatic it might be "like loosing your virginity to an Irishman". Another way it can be said with similar meaning is "like having sex with an Irishman".

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.