Eochaid ab Rhun (fl. 878–889) was a ninth-century King of Strathclyde, who may have also been King of the Picts. He was a son of Rhun ab Arthgal, King of Strathclyde, and descended from a long line of British kings. Eochaid’s mother is recorded to have been a daughter of Cináed mac Ailpín, King of the Picts. This maternal descent from the royal Alpínid dynasty may well account for the record of Eochaid reigning over the Pictish realm after the death of Cináed’s son, Áed, in 878. According to various sources, Áed was slain by Giric, whose ancestry is uncertain and who then proceeded to usurp the Alban throne.
Heir to the Brythonic kingdom of Strathclyde and a claimant to the Gaelic throne of Alba, Eochaid was of mixed-blood. Indeed, the name Eochaid is Gaelic and may indicate his maternal descent from the Alpínid dynasty. It is uncertain if Eochaid and Giric were relatives, unrelated allies, or even rivals. Whilst it is possible that they held the Pictish kingship concurrently as allies, it is also conceivable that they ruled successively as opponents. Another possibility is that, whilst Giric reigned as King of the Picts, Eochaid reigned as King of Strathclyde. Eochaid’s floruit dates about the time when the Kingdom of Strathclyde seems to have expanded southwards into lands formerly possessed by the Kingdom of Northumbria. The catalyst for this extension of British influence appears to have been the Viking conquest of this northern English realm.
According to various sources, Eochaid and Giric were driven from the kingship in 889. The succeeding king, Domnall mac Custantín, was an Alpínid, and could well have been responsible for the forced regime change. The terminology employed by various sources suggests that during the reigns of Eochaid and Giric, or during that of Domnall and his successors, the wavering Pictish kingdom—weakened by political upheaval and Viking invasions—redefined itself as a Gaelic realm: the Kingdom of Alba.
Eochaid is not attested after 889. Likewise, nothing is recorded of the Kingdom of Strathclyde until the first quarter of the next century, when a certain Dyfnwal, King of Strathclyde is reported to have died. Whilst the parentage of this man is unknown, it is probable that he was a member of Eochaid’s kindred, and possibly a descendant of him. A daughter of Eochaid may have been Lann, a woman recorded to have been the mother of Muirchertach mac Néill, King of Ailech.