Thanks, Martin. Given my background, and the fact that there were many Irish speakers in my life, it seems odd to me that I didn't know this one. I would have guessed at sliabh, cnoc, fa/na, or some other derivation from mountain, mount or hill. Live and learn is good sauce. I knew that reek is not from an Irish word for mountain, but apparently it is indeed a colloguialism of sorts. Thanks to all for the education.

All the best,

Mick