Is there a difference between Scottish Gaelic and Irish Gaelic and, if so, what is it?

Yes, these are two different languages, having diverged from a common ancestor about 1500 years ago (about the same amount of time that English and Frisian have been diverging from one another). It would take a sophisticated grammatical treatment to describe all the differences in grammar and phonetics that exist between Irish and Gaelic. In addition, the two languages adopted writing after their separation, so there are differences in the writing systems as well. For example, the singular personal pronouns (I, thou, he, she) in Irish are m�, t�, s�, and s�; the same pronouns in Gaelic are m�, t�, �, �.