Clegyr Boia, or Clegyr Fwya, is a prehistoric site on the St David's peninsula, Pembrokeshire, Wales, 33 to 65 feet (10 to 20 m) above the surrounding area.^[1][2][3] It is about 1 mile (1.6 km) west of St Davids, and about 0.75 miles (1.21 km) east of the Pembrokeshire coast. The hilltop's rocky outcrop contains evidence of Neolithic and Iron Age settlements.^[4] The site's nearest water source was Ffynnon Lygaid, a 1 foot (0.30 m) deep pool, fed by a spring. Near the base of the outcrop's southern side, the pool is in a small hollow, measuring about 1 foot (0.30 m) by 8 inches (0.20 m).^[5] The site was excavated in 1902 by Sabine Baring-Gould, and by Audrey Williams, for the Ancient Monuments Inspectorate, in 1943.^[4][6]

The outcrop became the stronghold of a 6th-century CE Irish pirate named Boia, after whom (with clegyr (English: crag)) it is now called.^[4]

Evidence of Neolithic settlement in Wales is extremely uncommon. Megalithic burial chambers are normally the only Neolithic structures to survive.^[7] However, an excavation of the site by Sabine Baring-Gould, in 1903, showed the remains of rectangular and circular Neolithic dwellings.^[4] An associated midden has been dated to around 5500 years before present (BP), during the early to middle Neolithic.^[3][4][8] Sherds of Neolithic round-bottomed pottery, polished stone axes and animal bones were also discovered.^[4][9][10] Baring-Gould thought the pottery may have been wheel-turned.^[4] No hearths were found within the dwellings. However, "substantial" charcoal deposits were found on the site, indicating fire was used, and carefully managed.^[8] Samples from the area of Neolithic occupation (hut and midden), radiocarbon dated in 1973, indicated a date of around 2370 BP, the beginning of La Tène culture, at the early British Iron Age, but were considered unacceptable by the National Museum Wales.^[6]

The hill's steep sides were augmented by a single earthen rampart, to form a defensive structure known as a hillfort.^[3] The outcrop's southern part was further enclosed by stone walls, roughly rectangular, and measuring about 280 feet (85 m) by 100 feet (30 m).^[9] Built as a dry-stone wall, from stone rubble and earth, the ramparts stand about 2 feet 6 inches (1 m) above the enclosure floor.^[1][11] Revetted with stone blocks, the walls varied between 9 feet (2.7 m) and 15 feet (4.6 m) wide and were "faced externally with large slabs set on end".^[1][11] According to Baring-Gould, the enclosure's original surface was 2 feet 6 inches (1 m) below its 1903 level.^[11] The enclosure's entrance may have been through a tunnel.^[3] Radiocarbon dated samples from near the hillfort entrance indicate a date of around 1950 BP (1 BCE), the late Iron Age.^[6]