I really like W.A. Cummins, The Age of the Picts (1995) for a sensible - perhaps even low key - explanation of the spread of Gaelic into Western Caledonia (and by implication the Isle of Man) as a lingua franca rather than a political conquest or a movement of people. He captures the idea of those northern coasts and islands as belonging to a common area of trade and communication. Of course people would have moved around (and marriages would blur lines), but Cummins allows for a dynamic, organic process rather than one built on military conquest.

All this said, I am not an expert on the period, so I would be very interested in hearing from anyone who has read Cummins and sees his argument differently or who can let us know that his work is discredited.

The definitive book on Dalriada is supposed to be Studies in the history of Dalriada, by John Bannerman. However, it was published in 1974, so might be of dubious value at this stage. There's also what looks like a solid paper by Marjorie O. Anderson called "Dalriada and the creation of the kingdom of the Scots" in Dorothy Whitelock's Ireland in Early Medieval Europe: Studies in Memory of Kathleen Hughes, which was published in 1982. I've read some other essays from that book, and they're good. I'm not seeing a lot out there that's more up to date, oddly.

Hen Ogeledd, I have to admit, I've barely even heard of before, and I don't think I've ever read anything about the Isle of Man, pre- or post-Norman.

With any luck, though, someone who knows more about the area and period will be along shortly - and do post in the weekend's Saturday Research and Reading thread if you don't get anything useful in the meantime.