I haven't read all of the Tain, just snippets of it. a lot of what you mention depends on how much you believe the original foundation myths - I'm still undecided either way. but one thing is certain, and that is the people of western Scotland and north-eastern Ireland have been moving backwards and forwards for thousands of years

an interesting thing to look at is the distribution of similarly-styled neolithic and bronze age monuments across Ireland and Scotland - there's a definite correlation running from south-west Ireland up through Perthshire to north-east Scotland (the stronghold of the Picts). later on in Pictish times it's along this same route that christianity spread into Scotland. Perthshire is full of ancient names recalling the "Irish" influence

if you're interested I can try and dig up some references from my books, but I'm going to be away for a week from Thursday so it'll need to wait 'til I get back

Cheers
Andy S (andy at stravaiging.com)