The RN service record is in Ancestry’s database called UK, Royal Navy Registers of Seamen's Services, 1848-1939. You can access that individual database from the Card Catalogue in the dropdown Search menu, accessed from the top menu bar.
Or, if you do a ‘global’ search from the Home page for Richard McDernott (tick the ‘exact’ boxes), the record should come up near the top of the results.
