visit china as an Irish person living in South Korea

If you only visit Hainan Island (where Sanya is located), you can make use of Hainna 30-day visa-free access. Just register with a local agency, and provide hotel and return flight orders. Recent feedbacks also say it is not necessary to register with a local agency anymore, which needs to be proved anyway.

If you also plan to visit other destinations in the mainland of China, then apply for a visa in advance in South Korea.