thePeerage calls the place "Inverdorat, the Black Cove, Angus".
Genealogics says "877".
Tompsett says "Died: 877, beheaded" but also "Weir says he was killed in a battle against the Danes at Inverdorat, the Black Cove, Angus." (presumably "Britain's Royal Families, The Complete Genealogy" by Alison Weir, Pimlico, 1996.)
The Henry Project, quoting "Constantinus macKynat xv a. reg. et interemptus est a Noruagensibus in bello de Merdo fatha et sepultus in Iona insula."; it translates "Noruagensibus" as "Norwegians", which is fairly indicative but might have been used to describe all Vikings or "Norsemen".