Great White?

AFAIK, they do come quite close to Ireland at times(run up the gulf stream) and have been spotted both here and off the west coast of Scotland. The temperature wouldn't be that big a problem as they have a limited ability(rare among "cold blooded" animals, leatherback turtle another example) of controlling their body temp and keeping same a few degrees above background temp. The waters they swim in off the coast of South Africa have currents running up from antarctica. The mixture of these cold and warm currents help boost the availablity of food sources.

What a lot don't realise is that the Mediterranean has quite a large population of the critters and among the biggest to boot ( http://www.answerbag.com/q_view.php/9872 ). They follow the Tuna around and feed on the seal population too. Saw a great vid years back taken from an oil rig off the coast of Turkey. It showed a great white attacking and eating a dolphin. I also remember a professional diver was eaten by one off the coast of Italy.