Second, although the researchers built their ship using old Irish ship building methods, it was not an exact copy of the actual corrupts used by the Irish in Brennan's time. Most importantly the researcher ship is quite a bit longer, 12 meters long, and has more sales than an Irish ship would have had in the 6th century. Sure, this larger more powerful ship is able to make and 8,000 kilometer voyage. But this doesn't really prove that the smaller Caracas built in Brennan's time could do the same.
Third, those intriguing markings discovered in North America. Well, some of those markings may resemble an old Irish alphabet, but many others do not. For example, some of the markings look like pictures, images of a son. a hand, a tree, real organ, the old Irish script was an alphabet and did not incorporate pictures, only letters. So the markings were probably not made by the Irish, but by Native Americans who did carve pictures into stone.