The Harleian Chronicle for the year 877 notes:
gueith diu sul inmon (Gough-Cooper, a438.1).

Translation: Sunday's Battle in Anglesey (Dumville, 12).

The Annals of Ulster for the same year state:
U877.3 Ruaidhri m. Muirminn, rex Brittonum, du tuidhecht docum n-Erenn for teiched re Dubghallaibh (Mac Airt, 332).

Translation: Rhodri son of Merfyn, King of the Britons, came in flight from the dark foreigners to Ireland (Mac Airt, 332).

Rhodri Mawr ap Merfyn Frych was the king of Gwynedd (Bartrum, 637) and this is probably a reference to the same event, as Rhodri died in the following year (Charles-Edwards, 487). The `dark foreigners? was a common term for Viking raiders in chronicle sources. The exact location of the conflict is uncertain, but it was most likely a coastal region of Anglesey.

RCAHMW (Battlefields Inventory), Nov 2016

Bibliography

Bartrum, Peter C., A Welsh Classical Dictionary: People in History and Legend up to about A.D. 1000 (Aberystwyth, National Library of Wales, revised edition, 2009).
Dumville, David (ed. and trans.), Annales Cambriae, A.D. 682?954: Texts A?C in Parallel (Cambridge, Department of Anglo-Saxon, Norse and Celtic, 2002).
Gough-Cooper, Henry (ed.) The Harleian Chronicle: Annales Cambriae, The A Text from British Library, Harley MS 3859, ff. 190r?193r, online edition.
Mac Airt Sean and Mac Niocaill Gearoid (eds), The Annals of Ulster (Dublin Institute of Advanced Studies, Dublin, 1983).