Names which you mispronounced for years, before eventually realising...

Niamh and no one I asked seemed to know either!
Also, Arkansas. I thought it was like Kansas with ar in front, turns out it's ar-kan-saw.

I went to school with someone called Chevron...who pronounced it the same way you'd pronounce Siobhan

Isn't it how it supposed to be pronounced?!
... Damn

David Bowie

Well, the Bowie part anyway, pretty sure I'm okay with David.

I went to school with someone called Chevron...who pronounced it the same way you'd pronounce Siobhan

Well that makes more sense than Siobhan does.

Is it "bow" as in a knot or "bow" as in the front end of a ship? I could never figure it out.

How does it? Many members in the thread have already explained why Siobhán is pronounced as it is, most people are just more used to English pronunciation as opposed to Irish, which is why it seems strange. The "s" at the beginning is slender (slender consonants are ones with "i" or "e" written next to them, broad consonants have "a", "e", "o" or "u" next to them), slender "s" makes a "shh" sound. When there are two short vowels next to each other, one of them becomes the "glide" vowel, so one of the vowels is favoured in the pronunciation, in the case of "io", the o becomes the glide vowel, leaving the "i" to dominate, but it's a soft "i" (like in "miss"). The "á" means it is a long vowel, making the "awe" sound. The "bh" makes a "v" sound. Ergo, the name is pronounced shh-ih-v-awn.