I had a rad hysterectomy and radiation in 1998. Last year a colonoscopy and endoscope, all clear. Have ibs. What could be the reason for the lower right quadrant of my abdomen being distended.

Many things. It is reassuring that the scopes were normal. The surgery can cause scar tissue called adhesions. Ibs can also cause accumulation of gas. The surgery can cause some weakness of the abdominal muscles. As long as the tests (ct scans, etc) are normal, i would recommend trials of diet changes and probiotics and exercise.
Several things. The most concerning possibility is cancer recurrence. You don't mention why you had a hysterectomy and radiation, but i assume you had cancer. Other ideas: adhesions causing a partial intestinal blockage, diverticulitis, a hernia, even constipation. But you really need to see a doc soon, so that the worst possibilities can be ruled out (or caught early!) i'm interested to know what they find!
IBS. All that you really have in your right lower quadrant is your intestines and bloating is a known side effect of ibs. Glad to hear colonoscopy was clear!