Colonic Irrigation is a scam, and the claim of built up shit is exactly that, shit.
Your colon isn't like a plastic waste pipe that stuff gets stuck to over time. It's made up of a lining of cells that are constantly dying off and regrowing (like most of the rest of you). The average epithelial (lining) cell in the gut lives for about 3 days, so you have a totally new colon lining around twice a week. As Dani says, you'll just wash out your commensal gut bacteria which do a lot of your digestive work for you.
If you're feeling bloated, try cutting out wheat and dairy for a few weeks, you may well have a dietary intolerance.