This is a Neolithic tomb dating back to 3750 BC and is made up of a pair of large upright standing stones which are on both sides of the central chamber which has two capstones supported by a number of smaller stones. There is another upright stone is at the back of the chamber. The site is located off the A5 between Tearddur and Holyhead.
Link: Cadw - Trefignath Burial Chamber
Image: Trefignath Burial Chamber image courtesy of Cadw