I found this interesting article on pennyroyal. It mentions using dried, crushed leaves of pennyroyal to repel fleas.

I would leave it alone though.

Pennyroyal taxes the liver and can be fatal if ingested in large amounts. The wiki article on pennyroyal has toxicity cases, but most involve direct ingestion of either large amounts of leaves or the essential oil of the plant, which most sources agree should be avoided entirely.

There are safer options however.

I've had moderate success deterring sugar ants by spraying a peppermint-infused apple cider vinegar at their entry point after using crushed mint leaves as a barrier until they stopped entering.