[2][3] Through his mother, Gruffudd had close family connections with the Norse settlement around Dublin and he frequently used Ireland as a refuge and as a source of troops.

[4] According to Historia Gruffud vab Kenan, Gruffudd's mother was Ragnailt ingen Amlaíb, a granddaughter of King Sigtrygg Silkbeard and a member of the Hiberno-Norse Uí Ímair dynasty.

[4] An anecdote described here about Ragnailt ingen Amlaíb explained a prophecy in which an unnamed monk told her that great power would come to her son.

Gruffudd landed on Abermenai Point, Anglesey with an Irish force, and with the assistance of troops provided by the Norman Robert of Rhuddlan first defeated and killed Cynwrig ap Rhiwallon, an ally of Trahaearn who held Llŷn, then defeated Trahaearn himself in the Battle of Gwaed Erw in Meirionnydd and gained control of Gwynedd.

[3] Rhys had been attacked by Caradog ap Gruffudd of Gwent and Morgannwg, and had been forced to flee to St Davids Cathedral.

Gruffudd this time embarked from Waterford with a force composed of Danes and Irish and landed near St Davids, presumably by prior arrangement with Rhys.

The armies of the two confederacies met at the Battle of Mynydd Carn, with Gruffudd and Rhys victorious and Trahaearn,[3] Caradog and Meilyr all being killed.

[3] [cy 2] Gruffudd again took refuge in Ireland but returned to Gwynedd to lead the assaults on Norman castles such as Castell Aberlleiniog.

The Normans were obliged to evacuate Anglesey, and the following year, Gruffudd returned from Ireland to take possession again,[3] having apparently come to an agreement with Earl Hugh of Chester.

He met King Henry I of England who granted him the rule of Llŷn, Eifionydd, Ardudwy and Arllechwedd, considerably extending his kingdom.

The see had been effectively vacant since bishop Hervey le Breton had been forced to flee by the Welsh almost twenty years before, since Gruffudd and King Henry could not agree on a candidate.

Owain and Cadwaladr, in alliance with Gruffudd ap Rhys of Deheubarth, gained a crushing victory over the Normans at Crug Mawr, near Cardigan, in 1136 and took possession of Ceredigion.

The latter part of Gruffydd's reign was considered to be a "Golden Age"; according to the Life of Gruffudd ap Cynan Gwynedd was "bespangled with lime-washed churches like the stars in the firmament".

Gruffudd died in his bed, old and blind in 1137,[3] and was mourned by the annalist of Brut y Tywysogion as the "head and king and defender and pacifier of all Wales".

[8] With wife Angharad (daughter of Owain ab Edwin) he had:[9][1] The FitzRery family of Swords, County Dublin, who were prominent in Dublin politics and commercial life until the seventeenth century, claimed descent from Gruffudd: since he was born in Swords, and maintained close links with Ireland, the claim is not implausible.