There is no such word as "sgian" in the Irish language. Perhaps it's the Scots-Gaelic version of "scian", which means "knife"? If you're trying to say "black knife" (dubh means black) in Irish, it's "scian dubh". I know of no such rule that calls for the lenition of the word dubh in this case (I'm not aware of "scian" calling for the lenition of any word following it, but perhaps this is a dialectical thing that I'm unaware of). "Scian dubh" is pronounced "shkee-uhn doo", with a slight "wv" sound at the end of the "doo" that's rather hard to describe and also dependent upon dialect. You could certainly get away with just saying "doo". The emphasis of Irish words are almost always on the first syllable (unless accent marks dictate otherwise), so your emphasis here should be on "shkee" of the word "scian". If I'm wrong about the lenition, and you wish to pronounce "scian dhubh", you would say "shkee-uhn ghoo-wv". The "gh" sound is very subtle and difficult to describe. It's almost like a cross between a "y" and a soft "g" sound. If you wish to learn more about Irish pronunciations, the Pimsleur system is excellent, although it teaches primarily the Munster dialect.
