Itching skin is a common problem in Kerries. Here are some ideas to try.
1. Always use shampoos formulated for dogs, and try either a mild puppy shampoo or one with oatmeal. Francodex makes a wonderful oatmeal shampoo that works great on my dogs. They also make an oatmeal/flea & tick shampoo that's easy on the skin if fleas are a problem.
As a last resort, you may try a hydrocortizone shampoo.
Also, you might want to consider two baths a month instead of one (if you're using a benign shampoo). It's possible that all the brushing you're doing isn't getting something out of his coat that he could be allergic to (pollens, leaf molds, etc.).
2. Rinse the suds out, and rinse again. Even the tiniest bit of soap left in the coat can cause itching.
3. Try clippering before the bath, and use a #10 or #7 blade to prevent irritating the skin. Also, clipper the corner of the mouth cleanly. Food collects there and can cause irritation or worse.
4. Add a teaspoon of vegetable (or olive) oil to his food daily (or every few days) to prevent dandruff. My dogs sometimes get dry skin in the winter, and doing this solves the problem.
5. Consider trying "Pet Relief," a spray to stop "hot spots" available from pet supply catalogs. I don't use it often, but as soon as I see one of the dogs biting itself, a timely squirt of this product usually helps.
6. Consider changing his diet. He could have developed a sensitivity to something in his food. I've used Nature's Recipe (Original), California Natural, Happy Dog and Abady--all with good success in getting rid of the itchies.