What about the Dal Riadan's to the west of the highland boundary, they could sail by way of The Minch in the isles, past Caithness and the Orkneys down to the moray firth, and be at Dun Taoru within days (hours even?), no? Just speculating here.. Or you could imagine a crossing of the highlands straight into Pictland, a hard journey, no doubt, but similar things have been done before!
By the way, would it be possible to sail straight through from the firth or lorn to the moray back in those days?
Another scenario, would be the Alt Clut britons departing from the firth of clyde, and doing the same journey perhaps? Those are the most interesting ones to me
A joined army of both would be even better hehe.