I think a dot over the b is the same as spelling it bh - where it sounds like a v? If not, someone will correct me. e.g. placename Cobh = Cove.

And Si is the Irish and Welsh spelling of the sound Sh - which they often use to replace J, which doesn't exist in their alphabets : Sean (Shawn)= John, Sian = Jane and so on. Also compare Sean (Irish) with Jean (French).