"Meaning the Norman ''de Cogan'' of Cogan, a parish in Glamorganshire. This surname came to Ireland at the time of the Anglo-Norman invasion. Milo de Cogan was the first constable of Dublin and arrived in Ireland with Strongbow in 1170. He received, jointly with Robert FitzStephen, a grant of the whole of County Cork from Henry II."
Goggin(s), Goggan(s) and Gogan are names that have been suggested as being derived from Geoghegan. However, these are usually variants of the name Cogan. The outstanding figure in the history of the Cogan family is the first of them to come to Ireland, Milo de Cogan (d. c. 1183), who was Strongbow's right-hand man in the Anglo-Norman invasion of 1171. He was granted a huge area in Co. Cork by Henry II. Milo left no surviving son and the great territorial family thus founded was practically extinct as such by the end of the seventeenth century. Minor branches of it, however, survive up to the present day, usually under the name of Goggin and sometimes Gogan. In the sixteenth century the name was in the transition stage: the earlier Fiants gave as a rule Cogan, Cogane and Coggain, the later ones Gogan and Goggan. Among the Co. Cork place-names in the same source we find Goganrath and Gogganshill, the latter being also given as Knockgogan and a few years earlier as Knockcowgan. Keoghan, Keohan and Kohan all come from the Irish Mac Eocháin. Because of the similarity to Mac Eochagáin, there has been inevitable confusion, however this is a name peculiar to west Cork and is not connected with Geoghegan. Egan (and it's variants Keegan and Hegan) which is Mac Aodhagáin in Irish, is an unrelated name despite its similar appearance.