Constantín's family dominated Fortriu after 789 and perhaps, if Constantín was a kinsman of Óengus I of the Picts ( Óengus son of Fergus ), from around 730.

The sophisticated kingdom that had been built fell apart, as did the Pictish leadership, which had been stable for more than a hundred years since the time of Óengus mac Fergusa ( The accession of Cináed mac Ailpín as king of both Picts and Scots can be attributed to the aftermath of this event ).

* Óengus II of the Picts

However, it appears that Eadberht Eating made some effort to stop the Picts under Óengus mac Fergusa crushing Dál Riata in 740.

The long period of instability in Dál Riata was only ended by the conquest of the kingdom by Óengus mac Fergusa, king of the Picts, in the 730s.

In 744 the Picts acted alone, and in 750 Óengus may have cooperated with Eadberht of Northumbria in a campaign in which Talorgan, brother of Óengus, was killed in a heavy Pictish defeat at the hands of Teudebur of Alt Clut, perhaps at Mugdock, near Milngavie.

Æthelbald, who might have been allied with Óengus, the king of the Picts, took advantage of Eadberht's absence from Northumbria to ravage his lands, and perhaps burn York.

* Óengus I of the Picts ( died 761 )

* Óengus II of the Picts ( died 834 )

The legend surrounding Scotland's association with the Saint Andrew's Cross was related by Walter Bower and George Buchanan, who claimed that the flag originated in a 9th century battle, where Óengus II led a combined force of Picts and Scots to victory over the Angles, led by Æthelstan.

According to legend, in 832 A. D. Óengus II led an army of Picts and Scots into battle against the Angles, led by Æthelstan, near modern-day Athelstaneford, East Lothian.

Óengus mac Fergusa ( variants Onuist, Hungus or Angus ) was king of the Picts ( or of Fortriu ), in modern Scotland, from about 820 until 834.

Eógan's reign falls in the period when Dál Riata was invaded and conquered by the Picts under Óengus mac Fergusa.

It is likely that his reign ended as a result of his defeat at the hands of the Picts of Fortriu, led by Talorgan mac Fergusa, brother of Óengus, at " Cnoc Cairpri in Calathros at Etarlinde ".

The independent existence of the kingdom of the Cenél Loairn, and that of Dál Riata, probably ended in 736, after which time it formed part of the kingdom of the Picts, ruled by Óengus mac Fergusa.

In 733 the Annals of Ulster report that Dúngal profaned Tory Island by forcibly removing Bruide, who is presumed to be the son of Óengus mac Fergusa, king of the Picts.

Fiannamail is said to have had at least two sons, Indrechtach and Conall, who were killed in 741 at the battle of Forboros, perhaps fighting against the Picts led by Óengus mac Fergusa.

Kenneth MacAlpin's mother was probably descended from the royal house of Fortriu and his great-grand uncle, Alpín, had reigned as kings of the Picts until deposed by Óengus in 728.

According to legend, Achaius, King of Scots ( possibly coming to the aid of Óengus mac Fergusa, King of the Picts ), while engaged in battle at Athelstaneford with the Saxon King Æthelstan of East Anglia, saw in the heavens the cross of St Andrew.

Mongfind also occurs as the name of the Pictish princess wedded to Conall Corc, ancestress of the Eóganacht Locha Léin of Iarmuman and of a line of powerful Pictish kings, e. g. Óengus I of the Picts, but it appears this can be dismissed as a case of coincidental influences on the story associating Crimthann mac Fidaig with the Pictish king and father, called Feredach.

Royal figure, dressed like a late antique Roman emperor, on the St Andrews Sarcophagus, probably Óengus I of the Picts.

** Óengus I of the Picts, d. 761

** Óengus II of the Picts, d. 834

These Scotti, a Celtic, Gaelic-speaking people, first appear there as settlers from Ireland in c. 500 when Fergus Mór, king of the north Irish kingdom of Dál Riata, and his two brothers, Loarn and Óengus, expanded their lands into southwestern Alba.

A Pictish king named Alpín, whose father's name is not given in any Irish sources, or even from the Pictish Chronicle king-lists, is known from the late 720s, when he was defeated by Óengus mac Fergusa and Nechtan mac Der-Ilei.

It was written by someone of Óengus's learning and literary skill at Tallaght and there are strong indications that this was Óengus himself: first of all, the sources named by Óengus in the epilogue to the Félire ( see above ) would make more sense if these were the materials used for the Martyrology of Tallaght ; second, a number of saints whom the same epilogue claims to have included are found in the Martyrology of Tallaght, but not in the actual Félire.

An assessment of Óengus is problematic, not least because annalistic sources provide very little information on Scotland in the succeeding generations.

Other Gaels, such as Caustantín and Óengus, the sons of Fergus, were identified among the Pictish king lists, as were Angles such as Talorcen son of Eanfrith, and Britons such as Bridei son of Beli.

While kings might patronise great Saints, such as Saint Peter in the case of Nechtan, and perhaps Saint Andrew in the case of the second Óengus mac Fergusa, many lesser Saints, some now obscure, were important.

It is sufficiently clear that Óengus became a cleric, since he describes himself as such in the Félire using the more humble appellation of " pauper " ( pauperán and deidblén in Old Irish ).

These pagan seats of power are contrasted with the great ecclesiastical centres of Ireland which were flourishing in Óengus ' own time, such as Armagh and Clonmacnoise.

To similar effect, Óengus also holds up the example of Máel Ruain, who continues to offer support and comfort after his death, against that of the contemporary warrior-kings Donnchadh and Bran Ardchenn, whose strong exercise of power meant no such thing after theirs.

" Not satisfied with his hermitage, which was only a mile from Clonenagh, and, therefore, liable to be disturbed by students or wayfarers, Óengus removed to a more solitary abode eight miles distant.

This rests largely on a confused passage in Symeon of Durham's Historia Regum Anglorum, and it has more recently been suggested that the interpretation offered by Frank Stenton — that it is based on a textual error and that Óengus and Æthelbald were not associated in any sort of joint overlordship — is the correct one.

Óengus died in 761, " aged probably more than seventy, ... the dominating figure in the politics of Northern Britain ".

Carthage was of the Eóganacht Chaisil and son, or, more probably, grandson of Óengus mac Nad Froích whom Saint Patrick baptized.

It has been suggested that the St Andrews Sarcophagus was commissioned by Óengus to hold Nechtan's remains, although it is more generally supposed that the sarcophagus was for Óengus himself.

Earnwine's father had been an exile in the north after his defeat in the civil war of 705 – 706, and it may be that Óengus, or Æthelbald, or both, had tried to place him on the Northumbrian throne.

Óengus runs Cellach through with his " dread spear ", which has three chains attached to it ; these chains wound one of Cormac's advisers and blind Cormac in one eye.

Having interpreted the cloud phenomenon as representing the crux decussata upon which Saint Andrew was crucified, Óengus honoured his pre-battle pledge and duly appointed Saint Andrew as the Patron Saint of Scotland.

* Second, on a visit to Midir ( escorted by Lug, the Dagdae and Ogmae ), Fúamnach announced that she would drive Étaín away from him and employed a spell which she had learnt from her fosterfather: she conjured up a mighty wind which blew Étaín through the air for the duration of seven years, after which she came to Óengus ( Mac Óc ) in the Brug.

There are reasons for believing that Óengus was ordained to the office of bishop, a denomination which is first assigned to him in a list of saints inserted into the Martyrology of Tallaght ( see below ).

The literary effort most commonly attributed to Óengus is the Old Irish work known as Félire Óengusso (" Martyrology of Óengus "), which is the earliest metrical martyrology — a register of saints and their feast days — to have been written in the vernacular.

Alpín was defeated twice by Óengus, after which Nechtan was restored to power.

On 12 August 729 Óengus defeated and killed Drest in battle at Druimm Derg Blathuug, a place which has not been identified.

The 750 battle between the Britons and the Picts is reported at a place named Mocetauc ( perhaps Mugdock, near Milngavie ) in which Talorgan the brother of Óengus was killed.

The amount of information which has survived about Óengus compared with other Pictish kings, the nature and geographical range of his activities and the length of his reign combine to make King Óengus one of the most significant rulers of the insular Dark Ages.

Kingarth ( Cenn Garadh ) and Eigg ( Eic ) were described as " in Galloway " ( Gallgaidelaib ) by the Martyrology of Óengus, in contrast to Whithorn — part of modern Galloway — which was named as lying within another kingdom, The Rhinns ( Na Renna ).