Stolen vehicle then.
Logbook and vin plate off a genuine non-recorded written off vehicle, transplanted on to a stolen one, if it was stolen in the UK it wont be shown on the Irish system.
I understand your sympathy with the seller, but he bought it to drive, but hasnt used it on the road.....course he knows, or at least suspects.
Next check would be the engine number, if there's one left.
Here's the thing, if you buy it cheap and think it may be stolen (even if you dont know it def is) then you commit an offence of handling stolen goods - perversely, if you buy it thinking it might be, but it transpires its not, you commit the offence of attempting to handle stolen goods. Wouldnt touch it with a bargepole now.
If you buy it, its to drive, so chances are at some point you'll get stopped by the Gardai, thats the point it all goes wrong, as it will always belong to the insurance company who paid out for it. Exception being Police auctions, which guarantee title.
Out of interest, where do you see them advertised in the Republic? Have thought about it before never found a decent source. (non-stolen ones I mean!!).
Regards
Pete