Mikel Arteta made a few notable changes for Arsenal, as Calum Chambers, Emile Smith Rowe as well as Eddie Nketiah earned a welcome return in the side's group stage finale.
Nketiah had his side off to the perfect start after getting ahead of the last man 12 minutes in to prod over an on-rushing Gary Rogers with a late intervention unable to deny the forward.
Dundalk succumbed again as Mohamed Elneny showed quick feet around the six-yard area six minutes later to send a sumptuous drive past Rogers and into the top-corner.
Action shifted to the other end in the matter of minutes when an exchange of passes picked out Jordan Flores, who charged into the area before firing into far corner to reduce the arrears.
The Gunners could've made the result more comfortable towards halftime through the boot of Ainsley Maitland-Niles, but Rogers dived across to turn behind for a corner-kick.
Dundalk were forced to make a substitution at the break after an injury from Andy Boyle gave Brian Gartland a chance in the last line of defence.
Cedric Soares should've made it three for the Gunners from a free-kick midway in the second half, but Rogers pulled across to beat the effort away.
Arsenal grabbed a third before the last 20 minutes when Joe Willock picked up a layoff from Folarin Balogun to fire underneath the gloves of Rogers.
It was more for the visitors 12 minutes from time as a ball out wide found Balogun, who applied a composed bottom-corner finish.
Dundalk bagged a second in the last seven minutes after Sean Hoare leaped at the near post to direct a free-kick past Runar Runarsson.
There was time for more in stoppage time with Chambers tucking in from a corner, but was called for offside as the clash concluded.