Sean, spelt in its Irish form is: Seán. The accent on the a means the name is drawn out when you speak it, and a lot of names in Irish that begin with 'S' usually are pronounced with a letter h after the S - for example,
Seamus,
Sile,
Sinead, etc.
so that SEEN becomes SHEEN and then when you drag the accented a, the name sounds like
SHAWN. I don't know if I've explained that very well, but hope it helps.