Training - how well is your dog in obedience training? Why is she trying to keep the cats OUT of the yard? I'd confine her to the house if the cats are outside - maybe you can consider an enclosure to keep the cats from roaming and protected from the dog.
Is the dog chasing the cats? This is prey drive, and you can distract him with toys and teach him that you and toys are more fun than chasing cats. This may not work with high-drive dogs.
