Hello Lisa,
It's wonderful that you will be spending your honeymoon on the Emerald Isle!
There used to be a left luggage facility at Kilkenny Station, but I wasn't able to check whether it's still in operation. You could perhaps contact
Irish Rail to find out.
Locker facilities are indeed available on-site for large bags at Kilkenny Castle. Although I would also suggest
asking them directly if it would be ok to leave your luggage for longer than you need to visit the castle.
Another option would be to spend an extra night in Dublin and make it a day trip to Kilkenny. Is it something you would consider?
That way you wouldn't have to worry about your luggage. Another thing to keep in mind is that the train from Dublin Heuston to Cork doesn't stop in Kilkenny (you would have to take the Waterford line from Dublin and then take a bus from Kilkenny to Cork). The
Irish Rail website contains more information about the trains departing from Dublin.
Let us know what you think,
Cheers,
Antoine