Here's a link to a nice instructional website, teaching the use of the DADGAD-tuned guitar in Irish traditional music. Very near the top of the first page are guidelines on sets of chords that "go together" in various major, minor, and modal songs. The information is just as useful when playing in standard or any other tuning as it is for DADAGAD:

http://members.cox.net/eskin/DADGAD.html

Here's the recently-reopned Mudcat thread where I found this info:

Guitar in Irish Trad Music