<i>Longphort, dún,</i> and <i>dúnad</i> in the Irish annals of the Viking period
John L. Maas
Longphort first occurs in the contemporary Irish annals mid ninth century as a new term for Viking ship-fortresses. In early tenth it changed its meaning completely and came to be used royal marching camps, previously called dúnad. The evidence suggests that this change happened over short time. Dúnad became obsolete; longphort replaced dúnad; dún towns of Ireland.
Peritia