Or in the Brythonic Dyfnwal, Goidelic Domnall, and English Donald
King of Strathclyde ( - 915)
Born ? Died 915
According to The Chronicle of the Kings of Alba Chapter 28
And in his time Doneualdus king of the Britons died; and Duneualdus son of Ede king of Ailech ; and Flann son of Maelsechnaill and Niall son of Ede who reigned for three years after Flann.
It was the Scottish chronicler John of Fordun that first made the assumption that this was a reference to a single Donald, son of Aed mac Alpin, brother of Kenneth mac Alpin, and that therefore that this Donald mac Aed was "king of the Britons" and had been placed on the throne of Strathclyde to rule it as a sub-kingdom of Scotland.
However, Ailech is (or was) in County Donegal, Ireland; the Annals of Ulster tell us that Donald son of Aed king of Ailech died in 915, which makes sense in terms of the chronological sequence of entries in the Chronicle of Alba. The Scottish Donald mac Aed is supposed to have been alive well after that date; some sources have him dying in 934, others, erroneously as it seems, show him as king of Strathclyde in the early 940s.
Clearly therefore, the reference in the Chronicle of Alba to a Doneualdus king of the Britons must be a reference to an entirely different Donald, and what the chronicler is really telling us that there was a "Doneualdus king of the Britons" who died in the same year as "Duneualdus son of Ede king of Ailech".
This Doneualdus wasn't Donald mac Aed and certainly wasn't Donald mac Aed's uncle Donald II because he was dead in 900 and it also noteworthy that the Chronicle names him as simply Doneualdus, which means our contemporary chronicler did not know the name of his father. This makes it extremely unlikely that our Doneualdus was a Gael or had any connection with Donald II whatsoever or the Scots royal house.
Exactly who Doneualdus was and exactly how he came to power in Strathclyde is impossible to say with any certainty as there are no other historical references to him, but it is highly probable that he was a representative of a dynasty native to Strathclyde. Possibly connected with the previous dynasty of Strathclyde, whose last representative Eochaid map Rhun died in 889 and very probably with Viking connections given the increased Norse influence in Strathclyde following the fall of Alt Cluid to Ivarr the Boneless in 870.
The 'Owen son of Donald' who is recorded as having been the ruler of Strathclyde in 937 is therefore almost certainly the son of this Donald.
SOURCES
The Henry Project at http://sbaldw.home.mindspring.com/hproject/prov/donal000.htm
Donald II and Falsely attributed additional child by Stewart Baldwin
Quoting as his source B. T. Hudson, 'Elech and the Scots in Strathclyde', Scottish Gaelic Studies 15 (1988), 145-9.
The Annals of Ulster online at
http://www.ucc.ie/celt/published/T100001A/
Ann Williams, Alfred P. Smyth and D. P. Kirby A Biographical Dictionary of Dark Age Britain (Seaby, 1991)