This little beehive is found on the tiny island of Eilean Fir Chrothair. The cell is unusually small, and would have sheltered one person at most. Originally covered in turf, the cell collapsed about a 100 years ago. Since then someone has done a great job of restoring it. You can see a (not very good) photo of the cell prior to its restoration on page 182 of this article from the Scottish Antiquaries Society.
The nearby islands of Kealisay and Little Bernera had early Christian settlements; so perhaps the cell on Eilean Fir Chrothair was their place of retreat - someplace for a monk to get away from the hustle and bustle of Little Bernera.
Eilean Fir Chrothair is not easy to reach. It lies 500 feet off the shore of Little Bernera, so it is a bit far to swim to. A kayak or canoe might not work either, as it would be a difficult landing onto the rocks that surround the island. I got to the island in June of last year by chartering one of Seatrek's big RIBs - see the June 30, 2015 post. Here are a few photos of the beautiful little beehive of Eilean Fir Chrothair.