I think you must mean a colonoscopy. A colonoscopy is a procedure that takes about 20 minutes or so. It involves putting a flexible tube - the width approximately of a large thumb - through your anus. With the help of blowing air and pulling and pushing the tube in an expert fashion one can get into the right side of your stomach and look at the whole of your large bowel directly.
There may be some abdominal pain and discomfort but you are usually given an injection to make you a bit sleepy and to lessen the pain. There is no radiation involved but there are a few risks which the doctor who does it will explain. Alternatively, he might do a barium enema - blowing a radio-opaque liquid up your bottom and taking X-rays. As you are only 27 years - your doctor must have some reason for doing this test. An irritable bowel itself does not need any of these investigations. Perhaps your doctor is trying to rule out any other problem.