Trahaearn was a son of Caradog ap Gwyn, ruler of Arwystli (in the south of present-day Montgomeryshire, Wales), a small state, on the south-western border between Gwynedd and Powys.
The same year Gruffudd ap Cynan landed on Anglesey with an Irish force and, with the assistance of the Norman Robert of Rhuddlan, defeated Trahaearn at the Battle of Gwaed Erw in Meirionnydd, gaining control of Gwynedd.
However tension between Gruffudd ap Cynan's Irish bodyguard and the local Welsh people led to a rebellion in Llyn and Trahaearn took the opportunity to counterattack, defeating Gruffudd at the Battle of Bron yr Erw at Clynnog Fawr in Caernarfonshire, also in 1075, forcing him to flee back to Ireland.
[1] Gruffudd ap Cynan returned on a second campaign from Ireland with an army of Danes and Irish to become King of Gwynedd.
The two enemy armies met at the fierce and bloody Battle of Mynydd Carn north of St Davids.