An accent or diacritic in Irish. Pronounced FO-dah (short o). Literally means long. Looks similar to an acute accent in French. Changes the sound of the vowel it is applied to.

a (pronounced as in am) -> á (pronounced as in saw)
e (pronounced as in dead) -> é (pronounced as in day)
i (pronounced as in bin) -> í (pronounced as in bean)
o (pronounced as in dog) -> ó (pronounced as in hole)
u (pronounced as in bun) -> ú (pronounced as in moon)