Don't have a shamrock or a leprechaun, what no pot of gold either?? We sometimes don't go out of our box, if we did we could think of many stamps and papers that we have to use. This is not a typical St. Patrick's Day image or card, but you can easily make it into one! Many people collect frogs and carry them for good luck charms.
I simply added "Lucky" then the inside can read "to know you" or "to have you". Add Happy St. Patrick's Day if you like.