According to Alison Kinnaird in her book "Tree of Strings" the earliest
evidence of the triangular frame harp is carved on a Pictish standing stone at Nigg in Easter Ross Scotland circa 800 AD.No triangular framed harp is depicted elsewhere before these. The Scottish stones show a fairly large instrument- the harper is seated with the harp resting on the ground and the forepillar level with the top of his head Triangular harps do not appear in Ireland until the 11th century on the Shrine of St Mogue.This is a smaller instrument resting in the player's lap.One theory about the smaller size is that the Scottish harps were strung with gut which could be long while the Irish strung their harps with wire which needed to be shorter.The first refernce to a harp in Wales is in a 13th century Llyfr Du o'r Waun manuscript that records the ancient Welsh laws. The harp may have been there before the 13th century, but this is the first documentation of it. Triangular harps became popular in most Medieval European courts.
The "harp" found at Sutton Hoo in England was a quadrangular instrument with 6 strings- the Anglo-saxons probably played this type of instrument rather than the triangular harp.