At a dig site in Annagassan near Castlebellingham researchers discovered a huge fortified settlement up to 150 acres in size established in about 841 A.D. where Vikings built and repaired their ships, traded and undertook raids into the surrounding countryside.
"Dublin developed more as a trading town, this appeared to be more of a raiding town," historian Micheal McKeown told the BBC.
"From here they attacked inland, they flattened all the monasteries in County Louth, they went to Armagh three times in one year, they went as far as the Shannon, deep into Longford.
"So there had to be a great amount of Vikings here," he said. "I would estimate 4,000 or 5,000 Vikings here with up to 200 ships."
Test trenches dug at the site in August found ships rivets, pieces of silver used as currency, a slave chain and an axe head, all of Viking vintage.
"There's been a bit of a mystery about where exactly the site was located or what exactly the site consisted of, and antiquarians and historians and archaeologists have been trying to sort that mystery since about 1750," said Ned Kelly, keeper of Irish antiquities at the National Museum of Ireland.
"We've now absolutely confirmed the location and nature of the site. It's a very large site."