Green’s Dictionary of Slang

1. (US Und.) an Irishman.

1613		Beaumont & Fletcher Coxcomb III ii: Hold fair, shamrock!
1820		The Wonderful Monkey of Liverpool 1/1: You bandy-legged, pistol-shinned, shamrock! you long-tailed, beetle-browed, pig-faced scoundrel! Do you cut peoples faces off here? I will knock the guts out of your skull in one moment.
1949		Monteleone Criminal Sl. (rev. edn).
2012		G. Pelecanos (con. 1972) What It Was 257: Him and one of his shamrock buddies, they cut themselves out of an exercise cage.

2. (drugs) MDMA [the logo on the pills].

2002

Greater Manchester Police Police and Schools Liaison Programme 🌐 Street names [...] M and M’s, MDMA, Shamrocks, X, XTC, Adam.

3. (US Und.) an Irish gang.

2003

H. Hill A Good Fella’s Guide To N.Y. 95: ‘Shares grass with the shamrocks’ meant the Irish mob was an ally.