Linn Duachaill is a real place. Was a real place.
In the 7th century, as the story goes, St. Colman cast a devil from a dark pool in the river and founded a monastery at the site. Hence ‘Linn Duachaill’, the pool of the devil.
In 841, Vikings overran the abbey. They occupied the site until the early 9th century. It is likely that they eventually abandoned the area because the tidal flows leave the rivers unnavigable for several hours each day–not such a good thing if you need a quick escape.
After the Vikings left, the monastery was apparently not rebuilt.
The location of Linn Duachaill was lost for hundreds of years. It was only rediscovered in 2010. More information about the research leading to this discovery, and the excavation to confirm it, can be found on the Linn Duachaill website.