I am currently trying to write a song with a Gaelic feel to it for a string ensemble. I am not asking for exactly the perfect notes to use, I am asking which keys are the best, and what tempos. Thank you for answering.
Apart from the ubiquitous Ionian and Aeolian modes ( standard major and natural minor respectively), The Dorian and Mixolydian modes also feature often in Gaelic music. It seems that the keys favoured are D Ionian and E Dorian (both containing the same notes), and G Ionian and A Dorian, again containing the same set of notes. This is probably due to the tuning of the stringed instruments used in this genre of playing.
Tempos will depend on the tunes, so I'd check whether it's a jig, reel, or whatever, and go with the appropriate speed.
The diminished triad (off vii) seems quite rare, whereas the substitution of bVII is an oft-used ploy. As in G major, an F major harmony will appear.
Compound metres - either 6/8 or 9/8 (slip jig) - are frequently used.