"Fadas" are accents which indicate that the vowel is long. Omitting them (or inserting them incorrectly) can change the meaning of a word, for example 'Seán' (Shawn) is a common Irish name but leaving out the accent makes 'sean' (Shan) meaning 'old'. Fortunately most Irish readers are alert to the problem and can get by if they must, without the accent, but it is a bit like not bothering to cross one's 't's. I have found that quite a few languages use similar accents and they are a great aid in pronouncation once one is familiar with them.