A shamrock tattoo is an indicator of one's affiliation with the Aryan Brotherhood, a white supremacist prison gang, as stated by The Southern Poverty Law Center and Florida Department of Corrections. The AB is also called "The Rock" in reference to the shamrock tattoos worn by the gang's members.
In Irish culture, the shamrock itself is a symbol related to the patron saint of Ireland, St. Patrick. Legend states that St. Patrick used the three leaves of the shamrock to show the creed of the Holy Trinity. The three leaves also held great significance in Celtic religion, in which three was a spiritual number.