The "Trainwreck" actress and director Apatow -- who were in Dublin to promote the new movie -- gave the bride and groom an unexpected surprise when they turned up at a local pub, reported Contactmusic.
Judd had arranged to meet local singer Glen Hansard for a drink at the bar, where it just so happened a wedding party was underway.
The trio gave guests an impromptu sing-a-long to the tune of Irish song "The Auld Triangle".
Guest Steve Cummins, who pictured Amy, Judd and Glen with the bride and groom, tweeted, "Who shows up for @jpswaine and Eithne's wedding only @amyschumer Glen Hansard and Judd Apatow (sic).