How to get a fada on an iPad or iPhone: hold your finger on the vowel you want, and a selection of accents will be offered - slide over to the accent you need.
On a Mac: option (alt) and the vowel you want. (And if you want an umlaut it's opt-8 then the vowel, a grave is opt-tilde then the vowel, a cedilla, opt-c then the c.)
On a PC: ctrl-alt-vowel.