You will find that Scots neo-trad bands use bass more and that Scots tunes are often easier to harmonize. Other answers have been given. I think the most important thing is to listen very carefully to what guitar players do on cds that have just a few players. Like, Arty McGlynn backing up a fiddle or flute. That will give you the feel without too much confusion.
The majority of the time, these guys do a lot of passing modal chords and partial chords so if you are playing bass, you would likely try and float along with em… you will often notice an absence of IV chords in root position, which is a bit confusing to “normal” Western harmony but its due to the nature of the guitar itself and the dropped sixth string D. They usually play a first inversion, like G/B e.g.Also, you may find a persistent but subtle chord substitute scheme on repeats of sections to make it more interesting, so if you are playing bass with someone, be prepared to play it differently.
No matter what, you will want to play nimbly, as most of the good guitarists do. The really good ones often use a very quick strumming style and are super creative in their accents (and you will want to meet them on those accents). For a very wide variety of substitute harmonizations and interesting touches, check out Solas. Its not pure trad of course, but they sure work around the melodies in an interesting fashion. Sunny Spells and Scattered Showers is the cd I am thinking of. Not everybody’s cuppa tea, tho.
We use a bass in our group and as I figure out the parts, I just follow the above guidelines, as I also harmonize most of the tunes. The best way I could characterize the bass parts in reels is that they are semi-“walking” bass parts that sometimes pause rather than play continuously. Because there are often several melody instruments of various timbres, you don’t HAVE to have a chord or harmonic stroke as much as you would expect in other styles. I often remind the guitar player to sit out on certain beats or just let the chord hang there.
And about every six months, I seriously doubt everything I have done to harmonize the reels and jigs, usually because I hear somebody great doing something completely different with the same tune. So my advice is, if using sheets to work out your ideas, do it in PENCIL. Don’t overharmonize jigs (once again, as I have done) but let 'em float a bit more. Hornpipes are easiest to add bass lines to.
And to re-emphasize, the harmonizations often used in trad do NOT fit in the expected Western Classical or even non-Irish folksong styles. They are wonderfully different and not predictable. Put simply, just because you see a triad spelled out in a melodic figure does NOT necessarily mean that you play that implied chord other than at the outset or cadence of tunes (thats how I got in trouble with jigs)… The melodies and harmonizations MEET at certain points in the phrase often… As you listen, you will see what I mean.
Though the use of guitar is knocked at times, the trad guitar players are ingenious, imo.
