They went further then Europe. Read Charles Hapgood's book, MAPS OF THE ANCIENT SEA KINGS. They travelers around the world and mapped Antarctica before it was covered with ice. All the world maps of the 16th, 17th. and 18th centuries. And the famous Piri Reis Map which was a copy of the one used by Columbus and showed most of Central and South America was made in the time of Alexander the Great.
Based on the article, the likely explanation is bunches of those snails were hauled to Ireland as a food source. What surprised me was that 8000 years ago traveling to and from Ireland was done at all, let alone routinely. Between real hazards at sea, and whatever ominous myths were in vogue, it'd take some mighty bold people to make that journey.