This is the last theatrical Droopy cartoon.
Although there is no Shillelagh castle, there is a small village of that name...about 2.5 hours by car from Shannon Airport (which was a popular stopover for refueling in the days before nonstop flights to Europe).
Final Droopy cartoon produced from the Hanna-Barbera era, lasted from 1943 to 1958.
The 1961 WB short "Compressed Hare" also features a giant magnet, with similar results.