How are we supposed to know? :x
Well, with regard to Seán, the fada (the line of the 'a') is of vital importance to the spelling, as it is what creates that "aw" sound in Irish. The "se" create the "Sh" sound. The reason the fada is quite important is because "sean" without the fada is a completely different word in Irish that means "old" and is pronounced as on would expect, "Se" creating an "sh" sound, and "an" making it rhyme with the English word "ban", creating the overall pronunciation "shan" (rhymes with the English words ban, tan, man, van).
In saying this, however, I have heard, but never seen, that in Northern Ireland "Seán" can allegedly be spelt as "Séan" but I must confess that I'm not really sure how this is handled but, to me, seems like a completely different word with completely different pronunciation as "Sé" makes a "shay" (using English pronunciation, rhyming with the English words "may" and "Day") sound, which would probably make it similar to the English name "Shane" rather than "Seán". I'm not the best at Irish, but "Seán" really should be spelt with the fada in order for the name's pronunciation to actually make sense, and to sound the same as "Shawn" because without it, while still referring to Irish origins, you're referring to a different word completely.