Drogheda United 1 Dundalk 3
Drogheda finished with 10 men as they lost for the seventh time in eight games and now look in real danger of automatic relegation.
Brian Donnelly's Boynesiders are only ahead of bottom side Bray Wanderers on goal difference, though with a game more played.
Ross Gaynor shot Dundalk into a 16th minute lead with Garry Breen doubling that eight minutes later following a free-kick.
Drogheda had striker Glen Fitzpatrick sent off just before half time following an apparent elbowing incident, though it appeared it was a case of mistake identity as it was John Flood who tangled with Wayne Hatswell.
Daren Meenan got a goal back for Drogheda on 57 minutes when Peter Cherrie couldn't hold Flood's shot and he tapped home the rebound.
Fahrudin Kuduzovic sealed the points for Dundalk on 88 minutes when firing home from substitute Steven Lennon's cross.