Well now there's a thing Gladman. See, I was taking the lead from the notice boards, understanding that they meant the whole of the two islands, and from greywether when he says "At 1894 feet [573 metres], this is said to be the highest surviving passage grave in the British Isles.", again meaning the whole of the two islands. But of course you're right, the Seefin passage grave is higher, at 620.6 metres to be precise. However, this: https://en.wikipedia.org/wiki/Slieve_Gullion#cite_note-10 says that the southern cairn is "the highest surviving passage grave in Ireland." and references this: http://www.mythicalireland.com/other/slievegullion.php and I'm understanding they mean Ireland, north and south.
So, all I can offer is, they're both wrong, so is greywether and so was I to repeat the error. Seefin is the highest.