Number #1: Consonant Mutation
The most notable feature of the Celtic languages, specifically, the Insular Celtic languages, is the consonant mutation. Different Celtic languages have different types of mutation, but a common factor is the soft mutation. This takes a “hard” consonant, and tries to make it softer by either shifting it closer to the voiced fricative spectrum on the IPA chart, or eliminating it completely. An example:
In Welsh, a language I have an acquaintance with, the soft mutation occurs in many different situations. However, the most common is the introduction to a definite article to a feminine class noun:
Cadair (Chair/A Chair) —> Y Gadair (The Chair)
Mam (Mom/A Mom) —> Y Vam (The Mom)
Geneth (Girl/A Girl) —> Yr Eneth (The Girl)
Different types of Celtic languages have different forms of mutation, so be sure to research them all to gain a true sense of variety!
Number #2: Plural Umlaut
Another defining feature of the Celtic languages, and Welsh in particular, is the act of plural umlauting. Umlaut refers to the changing of vowels within a word to make them closer in pronunciation to the following vowel. Tolkien used this language feature in his conlang of Sindarin.
This change in vowels was most likely due to the addition of a final i or y sound to the end of a word. The vowels within said word shifted to be higher or more fronted to match this sound. Example:
Bachgen (A Boy) —> Bechgyn (Boys)
Definitely be sure to research this further, as it is a cool feature to add to your own conlang!
Number #3: Voiceless Nasals
In both Irish Gaelic and Welsh is class of sounds rare in the spectrum of the world’s languages. The voiceless nasal. Though others may tell you different, we indeed have this sound in the English language. Hmm, I wonder where it is… There! In the “word” hmm. Instead of the “m” sound as in mom, which is made by vibrating your vocal chords, in the voiceless “m̥” the vocal chords are still.
The development of this sound came from the “nasal mutation”, which is a type of mutation akin to the one described above. If a nasal sound occured next to the voiceless “p”, “t”, or “k/c” sounds, then these phonemes would be influenced. “P” would turn into the labial voiceless nasal “m̥”, instead of the labial voiceless plosive. That “m̥” sound is the same as in “hmm”
All of these features can absolutely be utilized in your conlangs, or to inspire you to learn these beautiful and ancient languages!