By the late Roman Empire period, an Irish tribe known as the Déisi settled in the region between AD 350 and 400, with their realm known as Demetae. In the post Roman period, the Irish Déisi merged with the local Welsh, with the name of the region evolving into Dyfed. The region suffered from devastating and relentless Viking raids during the Viking Age, with the Vikings establishing settlements and trading posts at Haverfordwest, Fishguard and Caldey Island.
Dyfed, the region of Pembrokeshire, remained an integral province of Deheubarth but this was contested by invading Normans and Flemings who arrived between 1067 and 1111. But Norman/Flemish presence was precarious given the hostility of the native Welsh princes.
In 1457 Henry Tudor was born at Pembroke Castle and, 28 years later, landing an army not far from his birthplace, he rallied support, marched through Wales to Bosworth field in Leicestershire and defeated the larger army of Richard III.