Castle O'Dell was a location in the Fair Haven holodeck program, filename Paris 042. Situated atop a path leading from the town of Fair Haven, its battlement overlooked the distant city of Dublin. According to local legend, the King of the Faeries would reclaim the castle at nightfall, and any mortal who dined on his food would be transported to the spirit realm.
After discussing poetry at the Fair Haven train station, Michael Sullivan suggested to Kathryn Janeway that the two journey together to Castle O'Dell. He remarked that while he did not necessarily believe the superstitions surrounding the place, he had respect for them nontheless. (VOY: "Fair Haven")