In an effort to clear up my own mind on this subject, here is what I know from my copy of Donaldson (M.E.M. Wanderings in the Western Highlands and Islands)
The first authentic kingly burial on Iona is shortly after the demise of St. Columba viz.: the pictish king Brude MacBile. Also at the same time Egfrid King of Northumbria. There followed 14 of the early Scottish kings beginning with Kenneth MacAlpine and ending with Duncan and his murderer MacBeth and finally Lulach the Fatuous who died in 1057. After this day, Dumfermline Abbey built by Queen Margaret became the place for Royal burials.
Dunfermline is after Iona not before.