If Geoffrey of Monmouth can be introduced here hys book has the Picts coming to the Orkneys in the 3rd century BCE. This would mean that they came from the northern Irish Cruithne as distinct from t'other way about. On the other hand it is notable that the Welsh name of Prydein for Britain is usually held to come from being the island of the Pretani i.e. Picts. Some times I wonder if this 'Pictland' wasn't originally only the Orkney Isles, and there is some evidence of the Orcadians once having lorded it over parts of mainland Scotland.