Green’s Dictionary of Slang

(Irish drugs) a tablet of morphine sulphate.

1996		P. Howard The Joy (2015) [ebook] I’m also cooking up a shot. The auld morphine sulphate tablets, or napps as we call them [...] The napps are used to treat cancer patients and are every bit as strong as smack.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 122/1: napp n. a morphine sulphate tablet (MST).