Today (March 17th) is the Feast of St Patrick, an historical person who lived in the 6th Century and who is noted for his mission to Ireland. A number of legends have grown up around the saint and one of them supposedly accounts for the fact that there are no wild snakes in Ireland today. It is said Patrick drove them out.
However, most zoologists feel that there weren't snakes in Ireland before St Patrick's time. The reason for this is that the snakes didn't fancy swimming across. According to Wikipedia, Ireland became separated from Continental Europe about 16,000 years ago. The only other European countries without snakes are Iceland and Greenland.