Welsh, Cornish, Irish, Scottish, Manx, and Breton are all separate languages, but are considered "dialects" of the more vague "Gaelic" language group. These are also the members of today's Celtic League, and considered their own completely separate cultures. So yes, Cornish is its own language, but no, it's not "barely" related, as they're both different dialects of the same language group. They're like Italian and Spanish, which are both Latin languages; each Gaelic dialect is unique, but closely related to others.