Broad and Slender
Each consonant letter in Irish can represent two different consonant sounds, called broad and slender. This is a phenomenon unique to the Gaelic orthography, but with some parallels in other languages, Russian, for example. Vowels decide whether a consonant falls into either of these categories.
- A consonant preceded or followed by the vowels a, o, or u is a broad consonant.
- A consonant preceded or followed by i or e is described as slender.
In practice, this means that when a novice sees a word like "Lian", it's clear that the L is a slender consonant and the n a broad consonant, but you can't tell whether the vowel sound is an A and the letter I has been added just to show that the L is slender, or the vowel sound is an I and the A has been added to show that the N is broad. You must ask a native speaker, or use a dictionary showing IPA transcriptions.
Caol le caol agus leathan le leathan
The golden rule for spelling in Irish, caol le caol agus leathan le leathan means slender with slender and broad with broad. The rule says that the vowels on either side of a consonant (or group of consonants) should agree; they should both be broad or both be slender. The rule is primarily used 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. Note that there are a few common words (such as ansin and anseo) that do not satisfy this rule. However, the diphthong "ae" is considered broad, making words such as "Gaeltacht" and "aerfort" perfectly acceptable.