Gruffudd was born and grew up in Ireland, but, in this year, he crossed the Irish Sea with the explicit intention of regaining his inheritance in North Wales. He and his followers landed at Abermenai and quickly formed an alliance with the Norman Lord, Robert of Rhuddlan. Together they defeated and killed Cynwrig, Tra­haearn’s Lord of Llyn, before moving against the King himself. His forces were thrashed in Meirionnydd and Trahaern was forced to flee to his homeland of Arwystli. Gruffudd proclaimed himself King of Gwynedd. His first act, however, was to turn upon his Norman ally by laying siege to Rhuddlan Castle. He managed to take some booty but was unable to secure the castle itself.

Despite this show of strength, many Welshmen were unhappy at the large numbers of Vikings in Gruffudd’s army and the men of Llyn soon rebelled against him. Trahaearn saw the chance he needed and attacked Gruffudd’s army at the Battle of Bron-yr-erw, near Clynnog Fawr. Defeated, Gruffudd fled to Ireland once more. He did not stay away for long, however. In 1081, the Prince landed at Forth Clais in Dyfed, and merged his army with that of a new ally, Rhys ap Tewdr, another exiled prince who was trying to retake the Kingdom of Deheubarth. They clashed with Trahaearn and his men at Mynydd Cam. Trahaearn was killed and Gruffudd become the sole monarch in Gwynedd.

Triumph was short-lived. For, soon afterwards, Gruffudd was captured by the Normans at Rug, near Corwen, after being betrayed by one of his own men, named Meirion Goch. He was taken as a prisoner to Chester, while the Normans overran most of mainland Gwynedd, building castles at Caernarfon and Bangor, and on Anglesey, at Aberlleiniog. Traditionally, Gruffudd is said to have remained a prisoner for twelve years, although the exact period is unknown. He had certainly regained his liberty by 1094 when he played a major role in the general anti-Norman insurrection of that year. Four years later, however, the Normans forces in Chester and Shrewsbury made a concerted push against Gwynedd. Gruffudd withdrew to Anglesey and then, as before, to Ireland. He did return the following year, however, and the Normans tolerated his rule over Anglesey. Later, he was able to make himself “Lord of Gwynedd uwch Conwy” and consolidate his Kingdom while the Normans left hi undisturbed. King Henry I of England did lead a large army into Gwynedd in 1114, but Gruffudd lost no landholdings to him. Afterwards, the Welsh monarch did not fight in battle again, but allowed his sons, Owain and Cadwaladr, to extend the power of Gwynedd into Ceredigion, Meirionnydd, Rhos, Rhufoniog and Dyffryn Clwyd.

Gruffudd had married Angharad daughter of Owain ‘the Traitor’ ab Edwyn, Lord of Tegeingle, in 1095. They had daughters: Gwenllian, Marared, Ranill, Susanna & Annesta; and sons: Owain, Cadwallon & Cadwaladr. There was also an illegitimate daughter, Merinedd, who married Cadwgan ap Bleddyn, King of Powys.

Gruffudd became blind and decrepit in old age. He died in 1137, and was buried in Bangor Cathedral. An elegy about him was sung by Meilyr, his pencerdd. Gruffudd's wife, Angharad, survived him by twenty-five years.

The Welsh bards have always contended that it was Gruffudd ap Cynan who made certain regulations to govern their art. His name was certainly used to give authority to the statute drawn up for the Caerwys Eisteddfod of 1523. There is no corroborating evidence, but it seems distinctly possible that Gruffudd might have brought musicians and poets back with him from Ireland. Perhaps they influenced the development of the bardic organisation in Wales. It may be remembered that Gruffudd’s biography mentions his harpist having died at the Battle of Gellan in 1094. Gruffudd is the only medieval Welsh prince whose biographical eulogy has survived. In its earliest extant form, it appears to be a late 12th century translation of a lost Latin original.