Caol le caol agus leathan le leathan
Irish has a very regular spelling system. although it may seem intimidating at first. The golden rule for spelling in Irish, caol le caol agus leathan le leathan, means "slender with slender and broad with broad". It means that the vowels on either side of a consonant (or group of consonants) should agree; they should both be broad or both be slender. There are a few common words that do not satisfy this rule.
This rule is important when you add an ending to a word (e.g., when conjugating a verb). To satisfy the rule you may need to add a vowel between the word and its ending.
For more information, see: