I've always had this question about the pronunciation of Sean.
Is Sean a word from another language? Is it actually not pronounced Shawn and instead it's some sound between Shawn and Seen? Also, why isn't it pronounced as Sawn instead of Shawn, when the word Sean doesn't even have the letter 'h' in it to get that -sh sound?
Bottom line is, why is Sean pronounced Shawn instead of Seen?
Sean (written "Seán" or "Séan" in Irish) is a Hibernization of the English name "John"; that is, it's a transliteration of "John" into a form which can be pronounced in Irish and written with the Irish alphabet (which nowadays is simply a version of the Roman alphabet).
The Irish language does not have the sound /ʤ/ (the sound which English typically writes as "J"). It does however have a /ʃ/ sound (a "sh" sound in English orthography), which happens when an "S" is followed by a front vowel (in Irish, by an "i" or an "e"). Thus, in Irish, the letter sequence "se" or "sé" is pronounced something like /ʃɛ/ or /ʃe/ respectively.
With the "a" following, the name "Seán" is pronounced (if I'm remembering my IPA symbols and pronunciations correctly) /ʃɒn/, which is about as close to English "John" /ʤɑn/ as they can get.
Thus, when the name Seán/Séan began to be used as an English name, it was used with the standard Irish pronunciation, which sounded like "Shawn" in English, and later began to be spelled that way as well.
(Note: Someone can please feel free to correct my IPA symbols; it's been 30 years and I don't remember the pronunciations as well as I used to.)
Matt's answer here is close but off in a few regards.
The semi-Anglicised Sean is formed by removing the fada (accute accent) from the Irish name Seán.
It is a Gaelicisation (more specific than Hibernisation) of the Norman-French name Jehan which makes it cognate of the English John with both coming from the Old-French Jehan but in the case of the English the Anglo-Norman Johan then became John.
Irish is a much more phonetically consistent language than English, and so it's pronounced like Shawn because that's exactly how you'd expect those letters to be pronounced, an S followed by an i or e is pronounced /ʃ/ while eá is pronounced /ɔː/ and the n is pronounced /nˠ/ or generally /n/ by English speakers and indeed the /n/ sounds have merged in Irish use recently (and I have to admit, I can't tell the difference!).
The form Séan is pronounced Shan or Shane. This variant was once more popular in Ulster, but now one generally finds either Seán or the semi-Anglicised (by dint of dropping the fada) Sean throughout the island while the Anglicised Shane is similarly found in all provinces though still I think more often found in Ulster than elsewhere.
