I've never visited Barpa Nam Feannag, or even North Uist, but I saw shacmh's excellent photo, and couldn't resist finding out more.
From the information I dug up, the site is a 30 metre long chamber cairn. There's a stone slab to the east of it, and three large vertical slabs to the west. These are all thought to have been part of the chamber, the former possibly being a lintel.
For some reason the Multimap link doesn't work for this site, but the grid reference is correct (the eight figure reference is NF85677207).