Rob,
I would recommend reading the Wikipedia pages on Irish phonology and orthography to get a sense of when to use broad and slender consonants, as well as the pages on velarization and palatalization to figure out how to make said consonants. Other than that, I suggest just listen to native speakers of the dialect of your choice and practice, practice, practice. Try as hard as you can to imitate their speech.
And, yes, Irish spelling is more regular than English in certain cases, but there have been things that have changed. It actually ends up being much better if things are spelled in dialect instead of using the Caighdeán.
Jay Bee wrote:
d) vowels can lengthen or diphthong before l, n, r, m (and maybe ng, I'm not sure), so 'timpiste' becomes 'tímpiste', 'poll' becomes 'paull' etc
In Munster Irish, dipthongization happens at least before <nn>, as can be evidenced with their pronunciation of<ann>