Sligo Rovers also enjoyed a big win over Wexford Youths
Dundalk's outstanding week ended in a defeat at Galway United.
The League of Ireland leaders had made history by reaching the Champions League playoff round by beating BATE Borisov on Tuesday night.
But they fell to a 1-0 defeat at Eamonn Deacy Park, as Vinny Faherty's goal just before the break gave Galway victory.
He converted from a fantastic left-wing cross for Tommy Dunne's side.
Meanwhile, Sligo Rovers had a comfortable night away at Wexford Youths as they won 5-0.
Raff Cretaro opened the scoring after two minutes before Daniel Kearns (52), Achille Campion (67) and a Kieran Sadlier double gave Sligo a big win.
Goals from Brandon Miele and Sean Heaney gave Shamrock Rovers a 2-1 win over bottom placed Longford Town, while David O'Sullivan scored for the Midlands side.
Bohemians also enjoyed a win as Jake Kelly and Ismahil Akinade's second half goals saw off Finn Harps 2-0.
The goals in the 1-1 draw between Derry City and St Patrick's Athletic all came in the first half however, with Keith Treacy's equaliser cancelling out Rory Patterson's opener.