Trefignath is located on the island of Holy Island, which belongs to Anglesey. It is a megalithic structure of the Cotswold Severn Tomb type. The structure was excavated in 1987, revealing a three-phase structure within the remains of the mostly removed stone mound.

The oldest part is the western chamber, which was built over a cavity in the rock with access to the south. Four large supporting stones from it still remain. Radiocarbon dates from the old surface show that this structure was built around 3750-3500 BC.

The current middle chamber was built next. It is also still marked by four supporting stones. The mound was given a west-east orientation.

Finally, the far better preserved east chamber was built in the forecourt and the stone mound was extended to its current shape. The chamber is almost complete and has two large portal stones in addition to the capstones.