Óengus succeeded his brother Caustantín to the throne. Previously thought to have been of Dál Riatan origin and descended from Fergus mac Echdach, their family is now assumed to have been that of the first king Óengus mac Fergusa, perhaps originating in Circinn, a Pictish family with ties to the Eóganachta of Munster in Ireland. Óengus, along with his brother, son Eogán, and nephew Domnall, is included in the Duan Albanach, a praise poem from the reign of Máel Coluim mac Donnchada listing Máel Coluim's predecessors as kings of Scots, of Alba and of Dál Riata from Fergus Mór and his brothers onwards. The inclusion of Pictish kings from Caustantín to Eogán in the Duan led to the supposition that Dál Riata was ruled by Pictish kings, or rather that Dál Riata kings ruled Pictland, leading to supposition that the origins of the Kingdom of Alba lay in a Gaelic conquest of Pictland. However, it is now suggested that their inclusion is due to their importance in the religious communities of Dunkeld and St Andrews, where they were seen as founders and early patrons. However, a modern reconstruction of the later lists of Dál Riata kings presumes that Óengus's nephew Domnall was king of Dál Riata during this time. Óengus died in 834, the only event of his reign reported in the Irish annals, and was succeeded by his nephew Drest mac Caustantín. Óengus's son Eogán was later king and was killed with his brother Bran in a battle against Vikings in 839.
Association with Saint Andrew
in his Scotichronicon supplies a legend according to which Saint Andrew appears to Óengus II in 832, on the eve of a battle against the Angles. The saint advises the king to watch for the "sign of the Cross of Christ in the air". Bower's account has precedents in earlier chroniclers John of Fordun and Andrew of Wyntoun. A still earlier reference of the 12th century simply states that "King Hungus" won a victory aided by Saint Andrew, in gratitude for which the Picts agreed to venerate the Saint. Skene, in his notes to Fordun IV, xiii-xiv, states that the episode is placed in the 4th century, making the entire tale anachronistic in the extreme. The three kings whom the legend has been tied to are Athelstan of England and Athelstan of East Anglia, as well as Guthrum the Old. The "Cross of Christ" vision in Bower's account is turned into the white on blue Saltire in the narrative by George Buchanan, who has such a saltire appear in the sky in the form of a cloud formation during the battle. The religious site at St Andrews, originally Cennrígmonaid, long predates this Óengus. Túathalán, first known Abbot of Cennrígmonaid, died in 747, and it is thought likely that the establishment is due to the earlier Óengus or to Nechtan mac Der-Ilei. The St Andrews Sarcophagus is assumed to have been made for the remains of Nechtan or the first Óengus. The later St Andrews tradition recounting the supposed arrival of Saint Regulus at St Andrews, with relics of St Andrew, has him met at Forteviot by three sons of Óengus: Eogán, Nechtan and Finguine.
