(1) To screw someone over; scam.
(2) To inflict severe injury upon someone as leverage to facilitate the extortion of money.
"Shamrock" is most frequently encountered in the passive, as in, "you have been shamrocked." (Origin: Shamrock Towing, a notoriously unethical business in the greater Columbus, Ohio area, ca. 1957)
(2) "You didn't pay Vinnie your protection this month?! Buddy, you are sooo shamrocked!"