The case for catastrophe – the theories of myth revisited: Mythopedia
That's a lot of reading and I didn't read it all. I've seen this (below) interperted as ice bergs and offered as proof that Brendan took a northern route during his visit to North America.
"St. Brendan = St. Brendanus (m.)
• Brendan found a crystal tower in the sea and came to an island of giant smiths (Navigatio Sancti Brendani)."