Ua Riacáin, Muirchertach (Regan, Morice) (d. p. 1176), secretary and interpreter to Diarmait Mac Murchada (qv) (d. 1171), king of Uí Chennselaig and ‘of the foreigners’. He communicated the substance of a narrative on the Norman invasion of Ireland to the unknown Anglo-French author who subsequently redacted it into the verse chronicle known as ‘The song of Dermot and the earl’. Part of that narrative may indeed have been written by Ua Riacáin. Be that as it may, he was uniquely well placed to know of the events described, having long been a personal friend of Diarmait, ‘who held him dear’.
Nothing is known of Ua Riacáin's background or career other than what is given in the ‘Song of Dermot’. The work was written sometime early in the thirteenth century, some quarter of a century after Ua Riacáin compiled his material. It describes the events leading up to the invasion, from Diarmait's ‘rape’ of Derbfhorgaill (qv), wife of Tigernán Ua Ruairc (qv) of Breifne, in 1152, to the death of Strongbow (Richard de Clare (qv)) in 1176. It relates that in 1168 Diarmait dispatched Ua Riacáin to Wales with letters calling upon all potential Norman-Welsh allies to join him in recovering his kingdom in return for land and property. It is almost certain also that in 1166 Ua Riacáin had accompanied Diarmait to Bristol for talks with Robert fitz Harding, and that he also accompanied him on his mission to King Henry II (qv) in France. Again, in September 1170, Diarmait sent him to Dublin to call upon the Hiberno-Scandinavian ruler Asgall (qv) and the citizens to surrender the city to him and his Norman allies. It is probable that Ua Riacáin was a personal witness to many of the events of this span of twenty-four years. He would therefore have been of mature years when he passed on his information to the author, perhaps in the late 1170s.
The author's knowledge of Irish topography and placenames and his favourable opinion of Diarmait reflect Ua Riacáin's input and influence. The account of these momentous events is quite independent of the more prejudiced version given by Gerald (qv) of Wales. Ua Riacáin's date of death is not known; presumably he died some time after the last events described in the narrative – that is, some time after 1176.