There is quite a variety of definitions and disparity between them.
Insular Celtic is a term used to describe Celtic origins.
Insular Celtic covers Britain and the North West tip of France.
http://en.wikipedia.org/wiki/File:Celtic_Nations.svg Goidelic is the term used to link Irish, Manx and Scottish Gaelic.
Brythonic is the term used to link Breton, Cornish and Welsh Celtic.