Just discovered this wonderful pdf guide by Michael Eskin (in a discussion on The Session) - thought others here might benefit.  

If you haven't see it, it's a simple guide that can help figure out tonal center, mode & chords 'on-the-fly' for many traditional Irish tunes.  

Basic Traditional Irish Tune Backup pdf 11Mar2021  

The only thing that bothers me - small case Roman Numerals weren't used for minor chords (I'll have to ask why not).

I still think The MODAL CIRCLE of FIFTHS CHART comes in handy visually - for not only finding a tonic's scale or mode (I/i), but the neighboring keys are the the IV & V chords (easy I, IV, V major progression). 

I found some interesting info on an old Stack Exchange thread from Jerry Rockwell (dulcimer luthier/performer).  

Dorian  To modern ears, used to chord progressions, the signature chord sequence would be minor i - IV, and as with all of these modes, the bVII subtonic is used constantly to shift away from I, and to give a kind of cadence back home to I. (this is similar to how V7 cadences to I in tonal harmony, but it is a weaker tendency, and these modal progressions sound "folky" - the minor v is very much related to bVII, but in my decades-long struggle to understand pure diatonic harmony, I almost always use bVII, and throw in the minor v as a color chord here and there).  

Mixolydian  A signature Mixolydian progression would be I - bVII and this is found in tons of Celtic tunes, and in quite a few Grateful Dead jams as well. For the latter, I've always used I-bVII-IV-I.  I almost don't ever get sick of that!!  

Aeolian tunes are always harder for me to think of, but Black Is The Color and Poor Wayfaring Stranger are two that use the minor iv - which to my ear sounds just incredibly sad!!! (yes I know that's very subjective, but in our western cultures it seems to be somewhat agreed). The first four chords to Star of the County Down are one of my favorite Aeolian progressions: Bm - G - D - A, or i - bVI - III - bVII.

Jerry also mentioned this site:

'Songs In Irish'  

600 Songs in Irish Gaelic, Lyrics w/English Translation, YT Videos, AND CHORDS!!!