Stress

In standard Irish, most words are stressed on the first syllable -- capall is KA-pull, sasta is SA-sta. Easy enough to remember, since there are only a few exceptions for native Irish words
A few words (amach, tobac) are stressed on the second syllable. Any good pronouncing dictionary shows the stress on the word (and any secondary stresses that might be in place).

Dialectal differences, though, still trip me up. Munster Irish seems to drop the stress on the first syllable if the word begins with a- so the very common pronoun/preoposition combinations for at (ag) sound entirely foreign to me!

Standard, and Connacht Irish from Learning Irish, all prounounce agam and agat as U-gum and U-gat, swallowing that first a- sound (but, surprisingly, not doing the same for agus (and)). Pimsleur, being Munster-based, has ah-GUM and an-GUT, which sound abrupt and guttural to me. Stressing the 'g' like that

Important? Probably not critical - we all tend to shift the stress around words depending on the surrounding words, but this throws me off because it affects the pronunciation of that oh-so-common phrase for thank you -- go raith maith agat!

On the various tapes/cds, etc, that I have, it's pronounced variably as 'gurra-ma-UH-gut', ' 'gur-MY-ah-gut', 'gur-mah-AH-gt' and a few more. I have a tendency to mimic whatever particular version I hear (I'm a real jackdaw when it comes to pronunciation, I guess -- I have wto work very hard not to adopt a weird version of someone's accent when I talk to them.).