It's Shawn. I kind of want to ask the Seans I know whether foreigners or whatever call them Seen though.
To answer Foler, it is definitely "Shawn". No one with the spelling Sean, that I have ever been aware of, pronounces it "Seen." However, that dude with the fucked up zd in his name can go fuck himself, and you can tell him that, as someone named Sean, he is not welcome in our club.
Growing up, I was called "Seen" endlessly as a kid. It got significantly better in high school as the name became more common, but when I went to college, every foreign teacher said it wrong at first. I had one Electronics instructor from India that would hand out papers every day, and every day he called me "Seen", even though every day I corrected him.
To be fair, whenever I see the name Seamus, I immediately think "See-mus" but then I correct myself because I know it is "Shamus". However, I also think Sean is significantly more well-known nowadays, and most people look at it and hear it pronounced correctly in their head automatically just because of exposure. Seamus is still a pretty uncommon name outside of Ireland. Still, I get it, and don't get mad if someone says mine wrong the first time.
More recently, I placed a mobile order at Taco Bell a few months ago, and when I went through the drive-thru I told them I had a mobile order for Sean. The (black) girl taking the order said she couldn't find it, and asked what I ordered. As I started to list it off, she said, "Oh, here it is, under Sheen." I said no, that's how you spell Sean (pronouncing it correctly), and she said, "I don't think so." I opened my mouth to argue that I should probably know how I spell and pronounce my own name, but then realized I was probably talking to D'Niqua (pronounced Susan), so I just gave up and hoped for no spit or weave in my burrito.