Irish phonology has been studied as a discipline since the late 19th century, with numerous researchers publishing descriptive accounts of dialects from all regions where the language is spoken. More recently, Irish phonology has been the focus of theoretical linguists, who have produced a number of books, articles, and doctoral theses on the topic.
One of the most important aspects of Irish phonology is that almost all consonants come in pairs, with one having a "broad" pronunciation and the other a "slender" one. Broad consonants are velarized, that is, the back of the tongue is pulled back and slightly up in the direction of the soft palate while the consonant is being articulated. Slender consonants are palatalized, which means the tongue is pushed up toward the hard palate during the articulation. The contrast between broad and slender consonants is crucial in Irish, because the meaning of a word can change if a broad consonant is substituted for a slender consonant or vice versa. For example, the only difference in pronunciation between the words bó ('cow') and beo ('alive') is that bó is pronounced with a broad b sound, while beo is pronounced with a slender b sound. The contrast between broad and slender consonants plays a critical role not only in distinguishing the individual consonants themselves, but also in the pronunciation of the surrounding vowels, in the determination of which consonants can stand next to which other consonants, and in the behavior of words that begin with a vowel. This broad/slender distinction is similar to the hard/soft one of several Slavic languages, like Russian.
The Irish language shares a number of phonological characteristics with its nearest linguistic relatives, Scottish Gaelic and Manx, as well as with Hiberno-English, the language with which it is most closely in contact.