Eochaid came to the throne after the unfortunate death -- the unfortunate and violent death -- the unfortunate, violent and probably non-accidental death -- of his uncle Aedh. His claim descended, however, not through the male line (Aedh's predecessor, Constantine had a son Donald who held that position) but the female, via Constantine's sister, who had married Run, the King of Strathclyde.
On Aedh's demise, Eochaid seized the throne with Giric, a cousin, who was also descended from Alpin (and who, like Eochaid, may have known more about Aedh's death (unfortunate and violent) than history has managed to retain). Their joint rule was apparently less than ideal for Eochaid, and one version has it that he then collaborated with Donald to remove Giric from the throne.
Once he was king, in 889, Donald repaid Eochaid by relieving him of all his regnal troubles by sending him into exile in northern Wales, and obscurity, even the date of his death being now unknown.
Giric / Eochaid