A poem by Cynddelw Brydydd Mawr notes a victory for Llywelyn ab Iorwerth (d.1240) in his struggle with his uncle, Daffydd ab Owain Gwynedd, in 1194 (Insley, 166):

Ym Mon, bendragon, ban dreigiau --- Prydain,
Wyt prydfawr I?th arfau,
Cefeist a dofaist yn dau
Cad anhawdd y Coedaneu. (Jones and Owen, 241, line 21).

Translation:
In Anglesey, [the] main leader, head of the lords of Britain,
You are very handsome in your armour,
You triumphed and took the spoils
At the difficult battle of Coedanau. (Jones and Owen, 241, line 21).

The battle is also mentioned in a poem by Prydydd y Moch (Jones, 1991, 217, line 151). The Gwynedd chronicle O Oes Gwrtheyrn for the year 1194 also refers to the battle:

Or pan las Ywein hyd haf y Gwydyl vii mlyned ar wlwydyn rac wyneb y bu wrwydyr y Choedanau (Jones, 2013, 412).

Translation: From when Owain was killed until the summer of the Irish, seven years, and the next year was the battle of Coedanau (Jones, 2013, 418).

Coedanau is most likely identified with Coeden (SH 369 898), near Llanfechell in Anglesey, referred to as Coytdanew in the Extent of Anglesey from 1352 (Ellis, 57)

RCAHMW (Battlefields Inventory), Feb 2017

Bibliography
Elis, Henry (ed.), Registrum vulgariter nuncupatum "The record of Caernarvon"; e? codice ms to Harleiano 696. Descriptum (London, 1838)
Griffiths, John, 'Two Early Ministers' accounts for North Wales', Bulletin of the Board of Celtic Studies, 9:1 (1937), 50-70[ref Charles Insley, `The Wilderness Years of Llywelyn the Great?, Thirteenth Century England IX (2001), 163-73.
Jones, Owain Wyn, `Historical writing in Medieval Wales? (PhD thesis, Bangor University, 2013).