Is there any way to tenderize a beef roast after it has been cooked?

If you cooked an eye of round roast beyond medium rare, then it will only be easily eaten if it is sliced against the grain, and as nearly paper thin as you can do. You can reheat it gently in a broth or gravy, as KLS suggested, and do open face sandwiches. Your other option might be to cut it into very small bits and use in a pot pie or hash. Unfortunately, eye of round is just not a roast that will respond to longer cooking in the large cut format. You could also thin slice it and use it cold or room temperature in a grain bowl, top off a stir fry, or a salad. If you have a food processor, you can also make your own potted beef sandwich spread. It's not real gourmet eating, but it will help make a tough cut edible, and spare you wasting expensive meat.