I'd look into the following...
First, build trust.
You just got this dog, and you missed out on a lot of critical development time already since it is a year and a half old. No worries, however, you should take some time to build a trusting relationship with the young dog / old pup.
Second, teach it that the cats belong to YOU.
Use reward and correction to teach it that it should NOT pay any attention to the cats. Food rewards work well with most dogs. The correction simply needs to communicate that the behavior is NOT acceptable, and can be a simple verbal cue like "NO" with a little tug on the leash. The correction must be consistent, and strong enough to get the dogs attention and stop the behavior, but can't be so strong as to break the spirit of the dog, or injure it.
I'd start with the dog in the sit, and reward it for the sit. Introduce the cats into the dogs vision. When you see the dog look at the cats and start to move toward them, correct with a "NO", and when the dog starts to ignore the cats, reward with food and a "yes". Eventually, the dog will figure it out and leave the cats alone.
It'll take some time, however, with consistency, the dog will understand what you expect out of it and will comply.