The following information comes from a page put out by the Historical Research Centre. I have a photocopied version of this, but haven’t been able to find them on the Net to permission to re-use this material.
Ireland was one of the earliest countries to evolve a system of hereditary surnames. They came into being fairly generally in the eleventh century and indeed some were formed before the year 1000. The traditional belief that the system was introduced deliberately by Brian Boru is without foundation; it developed spontaneously in Ireland, as elsewhere, as the population increased the former practice, first of single names and then of ephemeral patronymics or nicknames proved insufficiently definitive.
The Irish surname Hannagan is patronymic in origin, being one of those names derived from the first name of the original bearer. The name is an anglicization of the Irish surname O hAnnagain, and means ‘the grandson of little Annadh’ or ‘the descendant of the little slow old man.’ The name is of uncertain origin, but it is possibly an old County Limerick surname and, in some cases is used as a variant of O’Hannon. It is mainly associated with widely separate areas, Counties Waterford and Tyrone. The earliest known reference to the name occurs in 1556 when John Hanigane of Waterford City obtained ‘English liberty.’ A generation later the name occurs among jurymen and trade guild officials in Dublin and also in Inishowen pardons. Later is was a principal name in the barony of Decies, County Waterford. In Dublin, James Hannagan was warden of the guild of Saint John the Baptist (merchant sailors) in 1752-53. In 1774 David Henegan founded the course for Cork students in the Lombard College, Paris.
Blazon of Arms: Quarterly gules and or, overall on a bend sable, three crosses pattee agent.
Translation: Gules (red) denotes magnaminity. Or (gold) denotes generosity.
Crest: A pennant sable, within a wreath gules, erect.
Motto: Turris fortis mihi deus
Translation: God is a tower of strength to me.
I must say I'm impressed with the family motto!