Or, if you have an Australian accent like me, the short "a" in m'athair "my father") is more or less like the "o" in "mother" and the long "á" in máthair is more or less like the "a" in "father".
This is the danger of comparing pronunciation to English. We don't all pronounce things the same. The way I pronounce "law" is probably closer to how you pronounce "low". IPA all the way!
Good point! That's also the strongest argument against a major overhaul of English spelling: as soon as you start talking about addressing the gap between spelling and pronunciation, you immediately raise the question: whose pronunciation? At least this way there's still one written standard (more or less) for lots of different spoken standards.