Most Americans (and Canadians) will insist on pronouncing all foreign words and names according to the most common English phonetics. Naturally, Caitlin becomes Kate-lin, shortened to Katie by her friends (you may find Cat-lin in the south. (Caithness?)
Sean is seen, (see-ann in the south), and is often presumed to be female.
I would guess that 90% of the people with Irish ancestry in North America have no idea how to pronounce Irish words. I am one of them.
APC, it took me awhile to get the joke. Liking dogs, the first thing that came to mind was Brittany spaniels (not a B. S. fan). Guiness is often pronounced Guy-ness. Perhaps because of the cowboy showman and entrepreneur, Texas Guinan. Which makes me wonder, what is the correct pronunciation?