It dates from 841, the same year Dublin was founded, and is believed to have been the previously unidentified fortress of Linnduchaill – one of two chosen by the Vikings when they decided to overwinter in Ireland.
The other location was what would become Dublin.
The locals in Annagassan have always known there had been Viking links but until the excavations, which began earlier this month, they had no idea how big or important those links were.
“Attempts to identify this site date back over 200 years and the significance of it is immense. It will be up there with all the major Viking sites in Europe,” said Ned Kelly, the keeper of antiquities with the National Museum.
He has a personal interest in researching the Vikings in Ireland and believes that Linnduchaill could have been “Dublin except (nearby) Dundalk Bay is shallow and access to it was determined by tidal conditions so Dublin won out”.
The discovery of the fortress, which is located on a stretch of land between the coast and the river Glyde, is especially exciting for archaeologists as it is on agricultural land and as such is “completely preserved,” he said.
The excavations have so far uncovered part of a human skull, rivets used to build and repair ships, silver used for weighting and exchanging, a spindle whorl for spinning thread and a brooch pin.
The €30,000 funding given to the Annagassan and district historical society to carry out the archaeological digs came from the Louth Leader Partnership.
Film-maker Ruth Cassidy, a member of the Annagassan historical society, said the group had walked up the river from its mouth and tried to think where they would have disembarked if they had been Vikings.
“This is where we came to,” she said, standing in a field, which was once a centre of international trade and where the Vikings built and repaired ships.
Louth county museum provided funding for a geophysical survey of the land and its results were so dramatic they were determined to secure funding to dig three trial trenches.
“The geophysical readings showed massive archaeological activity and we knocked on every door but were denied funding to do the trial digs.
“Then the society applied to the Louth leadership partnership and they gave us the money and thank God they did” said Ruth.
The excavations, by professional archaeologists, began three weeks ago and last Wednesday there was no doubting they had come upon something extremely important.
So far a defensive rampart consisting of a ditch and bank have been found along with other indications it was a main fortress built by the Vikings.