How short can you trim their beards? (Airedale, kerry blue, irish terriers)?

If your dog isn't a show dog, you can groom them however you want. Its up to you. If the beard is getting messy, just shorten it. I'm a groomer, and I do it for people all the time. One of my clients has a bouvier, and hey have long beards as well. This dog is always getting junk stuck in his face so she usually has me shave the beard off completely underneath and just trim around his mouth so its neat.