Stephen Kenny's side were beaten 2-1 at Tallaght Stadium
Dundalk midfielder Chris Shiels expressed his disappointment after his side allowed a one-goal lead to slip and suffered their first defeat in the Europa League group stages.
Two second half goals from visitors Zenit St Petersburg were enough to see them maintain their 100% record in Group D, while Dundalk continue to sit in second position on four points.
Speaking to Off The Ball's Nathan Murphy after the game, Sheils admitted that the result was disappointing but that there were positives to take out of the game.
"It was a bit disappointing, we probably see it as the one that got away," he said. "I suppose you have to take confidence out of the performance. They're a well established side in Europe and we showed we can mix it, but switch off and they will punish you. That's what happened."
The game constituted their fifth game in 12 days and they now turn their attentions back to their league defence when they host Bohemians on Sunday night. Their preparation for last night's game, he says, suffered because of the congested schedule.
"We didn't get a lot of time to work on tactics, you know? That's where the credit comes to Stephen [Kenny] and Vinny [Perth]. They have us so tactically astute going to these games. We knew that they were going to have spells of pressure and that we'd have to weather storms.
"We just had to dig in and rotate positions and it did work up until they punished us with the goals."
Dundalk are still to welcome AZ Alkmaar to Tallaght Stadium but their next test in Europe is a reverse fixture with last night's opponents.
"They are the best team we've played. They're an established side in Europe and they're more likely than not in the Champions League. They're the highest seed team in this competition, we're the lowest. That's where you do take confidence because we pretty much matched them for the whole game. We just came out on the wrong end of the result.
"But this is where we click back into domestic mode and we have to get the job done on Sunday. That's the main goal out of everything. We have the [FAI] Cup final as well so we can't get too carried away."