The meanings of the symbols in the dream should help bring your dream into focus. Here are links to their meanings:
monster (evil leprechaun):
dreamdoctor.com/dream-dictionary/definitions/monster.html
attacker:
dreamdoctor.com/dream-dictionary/definitions/attacker.html
attacked:
dreamdoctor.com/dream-dictionary/definitions/attacked.html
cat:
dreamdoctor.com/dream-dictionary/definitions/cat.html
Given the waking life background you provided (important for understanding the meaning of any dream), the dream appears to be about your boyfriend, who is symbolically represented in the dream by the leprechaun. Your dream clearly reflects your feelings of being under attack by him emotionally. Cats in dreams, besides being symbols for children, also represent a woman's sexuality. Without knowing what was happening between you at the time of the dream, it's hard to say conclusively why you were feeling attacked. However, when cute, harmless and/or fun icons (such as a clown or a leprechaun) appear to be the opposite (as in, menacing or evil) in a dream, it usually reflects the dreamers feeling that the nice, fun person they thought they knew has been perverted or overcome by baser, often sexual, instincts or urges. Could it be that your boyfriend has been making unwanted advances (read: attacking) or pressuring you sexually?
Feelings IN dreams are never disguised so how you feel IN your dream is how you feel about its subject in your waking life. Dreams about your boyfriend's behavior should never be scary. Clearly, you are afraid and uncomfortable in the dream. So, ask yourself, what is your boyfriend doing that is making you feel that way in waking life. Once you determine what that is, you need to either set up boundaries with your boyfriend or take a clue from your dream and run (i.e., end it.) Your dream is alerting you. It's better to acknowledge your feelings and take corrective action than to ignore them, if only because they will always find you in your dreams.
Hope that helps!