There are two main ways on Macs. The most hassle-free one is to use the shortcuts built into the system.
On any Mac, ALT + ` + vowel should give you lowercase àèìòù and ALT + ` + SHIFT + vowel uppercase ÀÈÌÒÙ. That’s pretty much it.
It helps to do this slowly initially until you get used to it. Press ALT (or Option as some call it) and keep it pressed down. Then press ` (just above ALT usually) and let go of both keys. OSX will remember you pressed them. Now hit any vowel and the system will put the grave over it.
The other option is to install Professor Carnie’s custom keyboard layout for Mac. If you install this layout, control-e + vowel will give you the accented vowels. It also gives you access to IPA characters, which might be handy if you’re doing Gaelic linguistics. More on this layout and the files you need here.