Dropkick Murphys -- No Luck O' The Irish ... Boston Airport Show Was Planned

Begosh and Begorrah ... turns out Dropkick Murphys' impromptu St. Patty's gig at Boston's airport had nothing to do with luck ... and it cost their fans a ton of gold to see it.

A spokesperson for Cara Group Travel tells TMZ the show was all part of the Celtic punk band's "Shipping Up To Dublin" promotion -- fans paid nearly $1700 a pop for airfare and tickets to see the band play ... in Ireland.

So, the 130 die-hards were genuinely shocked to get a bonus gig in the terminal before boarding their flight -- but everyone else ... the airport, airline, and TSA had prepped for it well in advance.

We're told the trip -- which sold out in 2 days -- is being filmed for a documentary about the group.