I know that pronunication is a painful thing to explain here, but I was curious if some of the more advanced speakers could put into "simple" terms (please don't give a bunch of linguistic babble that I won't understand) how to pronounce plurals that have internal changes like capall, gort, and casúr?

I know that the plurals are capaill, goirt, and casúir...but I have yet to find a good explanation of how to pronounce plurals that do this?

Not sure if this is going to help you in any way, but the trick is that (barring any dialectal irregularities) these plurals are pronounced the same as the singulars, except that the final consonant must be pronounced slender.

Now, what does it mean for a consonant to be slender? Say the name "Tatyana" - if you pronounce it like I do, then the first T is broad and the second is slender. The broad T is what you get at the end of the singular "gort", and the slender one is what you get at the end of the plural "goirt".

I can't think of any examples from English that contain a slender L (for "capaill") or a slender R (for "casúir") but I'm sure you get the idea: slender consonants sound "softer" than broad ones.

A good explanation (with sound files) of this whole broad and slender thing is here:

http://www.fiosfeasa.com/bearla/language/caol.htm