While it is often referred to as "Rosserk Abbey", it was actually Franciscan Friary. Founded circa 1440 and burnt 150 years later by Sir Richard Bingham, governor of Connacht during Elizabeth I's reign.
From where the cache is hidden, you can see the intricate designs on the stone doorway.
Make sure you re-seal the cache and hide it carefully. And also explore the Friary while you are here - it's a great place for playing hide-and-seek (though be careful not to disturb birds' nests in summer).