I think a dot over the b is the same as spelling it bh - where it sounds like a v?

It's a bit more complicated than that, I'm afraid! Yes - the "h" replaces the dot to which you refer. However, since O'Neill's day, there has also been a major simplification of Irish spelling and the word is now simply spelled "siúil". In effect, the "bh" was silent and was dropped.

In other situations, the "bh" sounds like either a "v", as you suggest, or a "w", depending on dialect.