It is believed that there was a land-bridge connecting the south-east of Ireland to the south of England – this existed before humans arrived in Ireland. I mentioned this previously in the post related to the birth of the Irish Sea – where during the Ice Age there existed a freshwater late before it was finally submerged by the sea.
It was 20 millions years ago that Britain and Ireland ended up where they are now. Almost 300 million years before that, the super continent Gondwanaland had began to separate in a north-south movement. Then 100 million years ago this shifted a an east-west separation, and by 40 million years ago the Atlantic Ocean was formed – and the continents as they are now were more or less in place 20 million years ago.