Linn Duachaill is the name of the Viking longphort near Annagassan. Viking activity in Co. Louth is documented in the Irish annals from the 820s and the foundation of a permanent settlement on the coast at Linn Duachaill is recorded in the year 841, the same year as the longphort of Dublin was established. For fifty years the longphort of Linn Duachaill was an important base from which raids were launched into the Irish interior and on Britain. In 891 the Vikings were expelled by the local Irish, however, after two decades they returned and Linn Duachaill appears to have been used as a base until around 927 when the fleet departed.
In its early phase Linn Duachaill seems to have rivalled Dublin as a Viking base on the east coast of Ireland. However, its location was not apparently favourable for the development of a town and it was eclipsed later by Dublin and finally abandoned.
To mark Annagassan's rich Viking history a Viking Festival is held on the second weekend in August each year.