No, the deer couldn’t swim over, that’s one reason this research was done. There’s nothing in the excerpt or the article about written records from the Irish Neolithic.
No Deer is an Island.
“By about 12000 BC plant cover began to appear in Ireland. For a thousand years, Ireland was a place of open meadows. Possibly still not an island, Ireland begins to take shape about 12,000 to 11,000 radiocarbon years ago (perhaps 11500 to 10000 BC). The following map gives a little more detail of the peninsula that would become the British Isles as the sea levels continued to increase.”
http://www.rootsweb.ancestry.com/~irlkik/ihm/ancient.htm