Simple, in the song sense, means instinctive and without thought, "free and birdsong" as the early collectors put it.
He elaborated on it in other arguments later.
Don't know what "simple" Irish music you mean - I gave up on the flute and the concertina after years of trying - far from simple.
A good musician if far from repetitious, we spent time recording musicians like Kevin Burke (fiddle), P.J. Crotty (flute), Fergus McTeggart (fiddle), Tom McCarthy (pipes, concertina, whistle and sometimes fiddle) and others talking about how not to make the music repetitious (ie boring).
Quite an education   
Jim Carroll