Certain keys (G major, D major, E minor) are easy on instruments such as fiddles and tin whistles. Certain modes (Dorian, Mixolydian) are also often used in Celtic music.
Songs to be played on pipes usually have a small range, as bagpipes can’t play very many notes.
Certain types of dances in certain time signatures (reels in cut time, jigs in 6/8) are commonly used in Celtic music.
Celtic music also uses a lot of grace notes (very short notes in between the main notes of the tune.) Bagpipes actually need to play these short notes in between notes that are supposed to be repeated, because they can’t repeat notes normally.
In sean nos singing, grace notes are used to denote emotion. Proper sean nos does not vary in volume - it would have been sung to be heard throughout a hall of outside gathering. So the grace notes are used for emphasis.
I’ve asked a very similar question before without a good response perhaps because I don’t know how to phrase it.
The easiest example of I can think of is in Fairytale of New York but also in many traditional-sounding tunes. It’s a certain high note that gets hit suddenly before the line goes back down to its normal pitch. In the aforementioned Fairytale, examples of this type of note are when the singer sing “place” (and perhaps “through”) in the line “But the wind blows right through you it’s no place for the old”.
Is this also something enabled by the key (since Fairytale was apparently released in D Major like Emily mentioned)?