The native speakers I talk to are from Galway, and they usually pronounce Siobhán Shoo-awn. I get in arguments with my brother's wife when I pronounce their daughter's name that way. Of course her people are from Kerry and are pronouncing the name it would sound in Munster. I was told Shawna is a corruption of Siobhan. BTW, the pronounciation of Kate-lin for Cáitlín is one of my pet peeves, but I rarely engage in that battle with other Americans. :-)

Brían