I'm not disputing this --- I don't know enough about such things to dispute --- but there was a window of time (roughly 10,000 - 7,000 BC) as the ice was retreating when there was a land bridge connecting the south-eastern tip of Ireland with south-western England, and connecting also England to the continent of Europe. Supposedly humans crossed the land bridge from Britain to Ireland in about 7,000 BC; then the rising sea level flooded the land bridges and, voila, the British Isles.
So why would there be snakes in Britain but not in Ireland, when the snakes had a couple of thousand years to complete this migration before the lowlands were flooded?
Got me. I’m just telling you what the zoologists say.
The British Snake Fauna is rather limited anyway. I think they have the common grass snake, Natrix natix, the common adder, Vipera berus and one other snake Coronella?. All small and not very very common. So, its not like England was ever crawling with snakes.
I do believe there is a small lizard found in Ireland.
One other thing about glaciation in Europe as compared with the U.S. In Europe, the major Mountain Chains run for the most part east to west. In America they run from north to south. Consequently when the glaciers advance in Europe, it was difficult for many species of plants and animals to survive north of the mountain chains as they were trapped against them as warmer climes proceeded southward. In America vegetation and animals coould easily move north and south with the advancing and retreating glaciers.