For me, the two best trad rhythm guitarists were always Mícheál Ó Domhnaill and Dáithí Sproule. Both DADGAD masters who seemed to know just the right chords to use and just the right places to change them to enhance the tune. There's a great old video of a concert of Mícheál playing with Kevin Burke on YouTube that's a masterclass in rhythm playing. The album "Traditional Music of Ireland" featuring Dáithí Sproule, James Kelly and Paddy O'Brien has some fantastic examples of Dáithí's playing.