At $300, just buy a harp in DUB and use it while there. Donate it to the right organization when you leave Ireland.

Other options are buying a seat for your instrument. When I worked for an airline years ago we frequently had seat baggage that accompanied passengers. Make sure the instrument is in a case.

Where do you live? You could contact a moving company and have a custom crate built for your harp, this is done all the time for fine paintings. The crate could certainly be checked and carried to your final destination. By carefully opening it you could then use the same crate on your return. Be sure the crate doesn't exceed the maximum checked baggage dimensions of 62 inches total - length x width x height.