Princess Royal of Picts, Unuisticc of Fortrenn (Uisneachdan)
1. The father of Kenneth MacAlpin of Scotland was King Alpin of the Picts (778 AD. -- 834 AD)
2. The father of King Alpin was Eochaid IV the Venomous, King of Argyll, (747 AD -- 819 AD).
3. The wife of Eochaid IV the Venomous was Princess Royal of Picts, Unuisticc of Fortrenn, born 755 AD.
The Picts practiced matrilinear descent. I believe Princess Unuisticc was Princess of the Royal family of Uisneachtan of the Kingdom of Fortrenn. The cognative qualities of the names are clear.
So where was Kingdom of Fortrenn? Fortrenn, or Fortriu, is the ancient area of Strathearn and Monteith in Perth. It is helpful to have a map to get an accurate perspective. Ancient Fortriu basically extends from the headwaters of Loch Lomond west to the Firth of Tay and includes within its boundaries Stirling and Dunblane and Loch Tay. McNitt's description of Fortrenn says most of it would be contained in present day Perthshire and Forfarshire. Within this area we find such places as Nechtansmere, Dunnichen, where in 685 the Pict army had a great victory over the Northumbrians, Abernethy where King Nechtan built a church in the early 700's, the river Nethy, all these placenames cognative to clan Uisnechtan, the Royal Sons of Uisnech.
Bear in mind that the first King Nechtan reigned 458 AD to 482 AD. The latter King Nechtan died in 732 AD. The Pict Kings moved their capitol from place to place. Sometimes it was in Scone in Fortrenn. King Nechtan's palace was on the river Ness according to Bede. The river Ness is about 5 miles long and extends from Loch Ness to the Moray Firth. If King Nechtan's palace was that close to Moray his clansmen must have occupied much of Moray. There are few or no Nechtan placenames in Moray. Here we have the King of the Picts located on the eastern end of Scotland's Great Glen, that natural rift that devides Scotland into two sections, a natural pathway from east to west. What do we find on the western end of the Great Rift? Answer: we find Dal Riada. Dal Riada occupies the lands which anciently belonged to the Sons of Uisnech. The MacHenry clan of Glencoe descend from Big Henry, Son of King Nechtan. We may surmise they may have retained their GlenCoe land by force against the Dal Riadans. But most of the Picts of that royal family, from the first Nechtan King (458- 482) had to move eastward along the Great Glen, and that is how they came to Moray and the River Ness. Plainly there was friction betweent the Dal Riadans and the Uisnechtan Picts. Ultimately a unification of the two peoples could come about through marriage. Hense Eochaid IV the Venomous of Dal Riada married Pictish Princess Unuisticc of Fortrenn and their son was the Great King Alpin of Alba.
Eochaid the Venomous of Dalriada...
I have never seen any explanation of why he was called "the Venomous". However -- he may have had to go to great lenggths to insure his son would gain the throne, which might necessitate the removal of any others who got in his way. It was largely a sexual thing. Well -- he was Eochaid IV which implies there were three Eochaids before him which passed the title from father to son, in an patrilinear system. And he married a princess of the Picts, who, according to Bede, used the matrilinear system. Of course this would mean a cultural war, a sexual war actually. Under the matrilinear system the most important person is the sister of the King or the daughter of the King. Not the son.
A good book on this is THE PICTS by Isabel Henderson wherein she elaborates to some depth on the probability that the Picts were a polygamous society. You see, a monogomous society is necessary if a man is to know for certain that he is the father of his son, in order to pass on the chieftanship or kingship for instance. But in a matrilinear society polygamous relationships are almost certainly necessary to make sure that a man does not know for sure if he is the father of a son, thereby preventing the possibility of a patriarchal society from existing. So if a King from a patrilinear society marries a Queen of a matrilinear society -- he might well resort to poisoning, or in any way possible murdering any other man who gets near his polygamous wife. He would be considered snakelike and venomous.
The list of Pictish Kings in the IRISH ANNALS contain no sons succeeding their fathers -- until two cases towards the end. So this problem did not exist in the early days. It was only when the Norse began to threaten Scotland that the Scots and the Picts began to realize that only by joining their kingdoms together might they be able to repel the threat. But mating a patrilinear system to a matrilinear system might have been something like mating cats to dogs.
We also have to take into consideration that Unuisticc's mother and sisters and aunts and their families may well have been very Christian, since King Nechtan was known as a pius Christian man, and if that is the case, it seems probable that they had already put polygamy far behind them by the time of Eochaid IV. Or maybe not. Maybe the Pict Christians stubbornly refused to abandon their ancient ways and found a way to include them in their version of Christianity.
At any rate, there are surely other possible explanations for the description "the venomous". This one just seems to stand out to me as one worth considering.