There are certainly dialects within Gaelic, and were it not for the last few centuries of decline, you’d find a range of dialects with differences much greater than currently exist in either Gaelic or Irish. East Sutherland Gaelic (the focus of Nancy Dorian’s seminal sociolinguistic research on language death) has only a handful speakers left (Ms. Dorian included), and it has enough differences with the dialects to the west to make mutual comprehension difficult (though there are other social & structural factors that affect this). I would imagine that other extinct or moribund dialects such as Perthshire, Aberdeenshire, and Galloway would also exhibit some major differences with the dialects currently in use.
Of the places where Gaelic is still widely spoken, you will find differences from island to island, district to district. But for the most part these are minor differences that don’t prevent communication. Lewis Gaelic (Isle of Lewis) sounds quite different from the other islands (even Harris), and a lot of people - especially learners - have trouble with it at first. Even for Gaelic, it’s very sing-songy, and tends to be spoken very fast with a lot of clipped words (at least that’s the common perception). Couple that with a fair number of unique vocabulary items and some pronounced differences in pronunciation (tinn (sick) is pronounced “chine” in Lewis, “cheen” elsewhere), and you’ve got a recipe for potential confusion. But with regular exposure, you adjust to the differences.
If you want a good example of Lewis Gaelic, listen to Coinneach MacIomhair’s program on Radio nan Gàidheal (available online).
Cape Breton Gaelic- there’s a topic near & dear to me! I learned Gaelic there, starting over 20 years ago. What a wonderful place and I highly recommend it for anyone, whether to learn Gaelic or just to visit, hear some fantastic music and see some amazing scenery.
But as for Gaelic dialects there, the general rule is that each community pretty well preserved the dialect from the place in Scotland from whence the settlers came. So you can hear Barra Gaelic in Iona and Christmas Island, Lewis & Harris Gaelic in the North Shore, and Lochaber and Small Isles Gaelic throughout much of Inverness County. There are some differences with Scottish dialects in terms of how each was influenced by the different varieties of English around them (you won’t hear Cape Bretoners bid farewell with tiorraidh, pron. “cheery” as in "cheerie-o), and there is a small amount of cross-dialectal influence in Cape Breton - the main example being the spread of the Glug Eigeach, in which broad l’s (i.e. an l bounded by the vowels a,o and/or u) is pronounced like a w. So the word latha, “day”, pronounced “laa”, is pronounced “waa”. This is common in the dialects of Lochaber and some of the Small Isles (such as Eigg, which is where the term Glug Eigeach comes from), and so was common across most of the west side of Cape Breton as well as Antigonish County on the mainland. For some reason this feature spread over to the other Catholic communities of Cape Breton, which were mostly Barra and South Uist settlements. So people in Boisdale and Castle Bay say “waa”, but in the old country, folks in Lochboisdale and Castlebay say “laa”. If you listen to the singing of Mary Jane Lamond, you’ll hear the Glug Eigeach clearly.
There is of course much more to say about Gaelic dialects, but I think this gives you an idea. In terms of how it impacts learning materials, in my experience the speakers on the tapes usually strive for an approximation of the perceived standard, as you would hear on radio and tv. The dialects don’t get erased, but they do get modulated to minimize the differences.
