I am a regular patron of Ri Ras, and know the staff well, I will check into the age limit question. I do know that Friday nights are very busy there, and there is a DJ on Friday nights, so there will be a cover charge. The place will be packed, and pretty smokey. Saturday nights is live music night, so again, a cover charge and a full house.
An option might be Sunday night. There is a Celtic jam, no admission, and no age limit. I go often,the music is excellent, lots of fun, and nowhere near as crowded.
Other places to consider...

- The Galway Hooker in Lake Norman.
- Sir Edmund Halley's, Park Rd./Woodlawn area.
- The Flying Saucer, University area. (though there might be an age issue there as well)