The word originates in from the Irish 'maith' which means good. Caillín maith meaning 'good girl' became a way of refering to your girl friend.
Over the years the word changed so it was simply 'mot', which became used by lower class Dubliners.
Now is widely used to refer to all women of similar ages and not just girlfriends.
Me mots coming over later to give us a wank.