Two of the six celtic languages are called "gaelic": There is Irish gaelic, sometimes called just Irish, and there is Scots Gaidhlig, also called just gaelic. They have a lot in common, but they also have many differences in spelling, grammar and vocabulary, and speakers of the two languages do not necessarily understand each other. (The other person in my scots gaidhlig class is a fluent Irish speaker, and he is able to use his knowledge to understand a good deal of what he reads, since he's clever and a natural linguist; but he is not moving faster to write or speak gaidhlig than I am really.) Scottish gaidhlig is the newer of the two languages, and actually comes from the older Irish; but they have diverged a great deal.
I guess a good modern index of a language's separate existence these days is whether they are translating Harry Potter into it.
The Irish translation is already in existence; recently plans to do the Scottish translation were set in motion, and the first volume should be out in a year or two.
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