I’m not exactly an expert, but here goes.
Thor Heyerdahl seemed to prove that at very least, travel from Africa to South America was indeed very possible, via the Ra Expeditions. He noticed that the reed boats used on Lake Titicaca in South America were virtually identical to ancient pictures of Egyptian papyrus boats, and set out to see if the papyrus boats could make the trip.
Darned if they don’t happen to be seaworthy as all heck, and capable of going one way, at least . . . don’t see why they wouldn’t be able to return.
Mind you, some of Mr. Heyerdahl’s other theories have been discredited, but that doesn’t undermine the basic fact he proved that it could have been done. Same as the Kon-Tiki expedition really did help spark a revival in traditional forms of navigation in Polynesia, as well as demonstrate anothe possible way for people to have settled there.
I think it’ll be a while longer before we see any real definite proof, apparently it just occurred to many that the most likely sites to find this evidence are underwater. Coastlines have changed dramatically over the last 5,000 years or so . . .
As for Saint Brendan, he got his directions off the fishermen who’d been fishing off the Grand Banks for generations. 
There is also some question about a Prince Madoc of Wales, links here and here. However, apparently there’s considerable debate whether or not he actually existed. And as for the second link, I also heard somewhere that the Mandan tribe of First Nations (along with the Lummee) exhibited Caucasian characteristics because that’s where the Roanoke colonists really went. Take yer pick.
And general comments on seaworthiness of ancient vessels - when you hear about dugout canoes, I bet most people think of the crude, hollowed-out log type. Check these out, especially the ones on the left, then look at this one. I don’t see why they couldn’t be seagoing rather than just coastal. And if you follow the Aleutian island chain down from Alaska, there you are within spittin’ distance of both Russia and Japan.
Then there is some evidence suggesting there was ancient Japanese trade on the West coast of South America (anchor stones found in, I think, San Francisco Bay). The coast Salish had full body hair, unlike most First Nations. There’s a theory that the Salish are/were related to the Ainu of ancient Japan.
In other words, most anthropologists are pretty sure it occurred, but definitive proof is pretty thin at the moment.
I wanna know how the possible Australian Aborigine made it to Tierra del Fuego. To nutshell this one, and I REFUSE TO GET INTO THE GREAT DEBATES ON RACIAL TYPING I AM PERFECTLY AWARE OF THE PROBLEMS WITH THIS SORT OF THING, the skeletal charateristics of an ancient skeleton found in Tierra del Fuego most closely resemble the Australian Aborigines. Neat.
Anyways, this post is awfully long and I bet you’re tired of the lecture anyways.
:looks around, sees empty room, slinks off:
Tisiphone