Mac (OSX)
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.
Another way
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.
There’s a third way now… recent versions of macOS support a similar accent system to iOS and iPadOS: if you hold down the vowel key, you’ll see a pop-up showing variations on the letter; once it’s there, press the number next to the variation you want. So è can be made by holding e, then typing 1; for ò it’s o-3.
Of course Option-` is faster, but this is a good trick to know for other accents and variations.