nice story, the one of Troy. I might add, that this very Sinon had toi convince the Trojans not to break down the horse, but the gate for it would not pass through it because of it's heigth. Afterwards, it was late already and the Trojans decided to go to sleep, instead of first rebuilding the gate.
About the name "Sinon": you might be right about different sounds from the same source. As far as i know, Sinon may have been derived from Shannon, which is the Cork river nr.1 as you probably know.
My family comes from Bruff (Limerick) and can be traced down as far as 1440. Guess it's an ancient name already.