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

3/18/2015 1:45 PM PDT

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.

Erin go bragh!