I agree that you probably don't need to know all that mode stuff. I studied all the early church modes & much of the early plainsong, etc. as part of my Music History & Music Theory in Music school while at college,---so yes, I can tell by looking at the music, &
listening to tunes what mode, what key, whether major or minor, etc. The average person does not, but so what? There are some really funky minor modes in Irish (Celtic) & such Music. Do not atempt to ascertain the key by looking at the key signature. Ya just gotta listen to the tune & play it. The guitar player may need to understand it a little, unless a good Musician has taught them the correct chords--then they can just play & have fun. Alot of the time, the tunes you can get off of some websites have a bunch of chords changing all the time, with chords that don't even sound close! Guitarists should listen carefully to the tunes & try to stick to I, IV, & V (yes, V & not V7) most of the time.
    Sometimes you may wonder why a certain #,b, or natural is not in the key sigature. There are some modes which are sorta pentatonic-like & may leave out one of the notes of that particular scale, like the do-re-mi---ect.
    For example, "Amazing Grace" is normally in the key of G major--one sharp, F#--according to the key signature (in almost all hymnals & also as played by most "traditional" bluegrass folks), but the melody does not use the F#. That is normal for some "Celtic" Music tunes. So what does that mean to us? Really, not that much to folks in a jam/session. They play tunes as they have learned them. If you listen, & get it, just play the tune & don't worry about all the rest of the Theory junk.
    I grew up playing in Ceoli bands, & went through the various tunings, including dadgad & lots more. The more I played, the more I used regular guitar tuning. Now I don't need no dadgum tunings.
There are many times when I need to play in various keys. For some of the funky modes I usually play regular I, IV,V7 chords or the minors, only sparingly using the 7ths as in the V7 chord. That way it won't clash against the melody of a funky mode tune. The longer I play, the more I find that I prefer that real traditional sound.
    Try listening to some good Traditional Musicians playing Traditional Music in the Traditional style & get the chords from them. But, to each-her or his own.
    Good Luck!