aka Eochaius, Achaias or Achaisas,
Grandson of King Aedh Finn, warlord and sub-king of Dalriada (Kintyre)
According to legend, Achaius established an accord with Charlemagne of the Franks (and thus the origins of the "Auld Alliance") and commemorated the alliance by founding the Noble Order of the Thistle in 809 (See Ashley, p. 206 and Wikipedia "Order of the Thistle.")
Eldest Son of:
King Eochaid IV "The Poisonous" (ca. 755 - 805), r. King of Dalriada ca. 781 - 805?
Born: ca. 782
Married: ca. 805
Fergusia aka "Urgusia" dau of King Angus II of Dalriada (who reigned 821 - 834)
Died: Bef. 834
Sources:
Ashley, Mike. The Mammoth Book of British Kings and Queens (New York: Carroll & Graf Publishers, 1998) p. 206
Moriarty, George Andrews. The Plantagenet Ancestry of King Edward III and Queen Philippa (1960) pp. 29, 278
Paget, Gerald. The Lineage and Ancestry of H.R.H. Prince Charles, Prince of Wales (Edinburgh: Charles Skilton, 1977) p. 151