How is Seán pronounced in Ulster? Someone told me that it was not Shawn but like sean(shan)

In much of Ulster, the "á" has a flat, nasal sound to it (if you listen to a recording of Altan, you'll hear it clearly in the way Mairéad sings "tá" and "grá"), so while I haven't heard it personally with Seán (I don't know a lot of Seáns, apparently) I wouldn't be at all surprised.

Redwolf

Yeah, it's between an ah-sound and an ay-sound, more or less the same sound as in "bad" in English.

http://www.forvo.com/word/se%C3%A1n/#ga
(the 2nd recording)

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GRMA

Isn't that why it's transcribed in English as Shane rather than Shawn?

I guess so.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Sometimes it is, sometimes it isn't. There are many "Shawns" and "Shauns" in the U.S. But yes...someone who took the name from the Ulster pronunciation would likely transcribe it as either "Shane" or "Shan."

Redwolf