From the experts:
The name of several ancient kings and princes of Ireland, Tadhg became so common at one point that it was used to represent a kind of Irish Gaelic everyman, or man in the street, as Paddy and Mick would later. Tadhg has seen a major resurgence in recent years and is also now ranked in England. It is sometimes used as the Irish equivalent of Timothy and is also anglicized as Teague and Thaddeus. Tadleigh and Thad are pet forms.