Nicholas Kuhn and Reo Hatate threatened early on for Dundee, the former twice forcing goalkeeper Jon McCracken into action.
Dundee weathered the pressure and Amadou Bakayoko diverted a cross into Joe Hart’s grasp after being unable to get much purchase on the shot.
It was just at the point the hosts were starting to gain momentum that Celtic scored as Kyogo Furuhashi’s flick set up for Forrest to strike across McCracken.
The home side showed renewed vigor after the break and Mo Sylla headed a corner wide and Jordan McGhee nodded another at Hart.
But, again, Celtic showed their quality in the final third.
Reo Hatate passed forward and Ricki Lamie and Portales got in each other’s way, leaving Forrest as beneficiary and scorer. He soon made way to cheers from the away fans.
And Celtic could have added to their lead, the graceful Hatate checking and sweeping a shot on to McCracken’s right-hand post.
Dundee finally profited from a corner, albeit with a degree of fortune.
Portales’s downward shot did not seem to have the power to cause Hart problems but he was wrong-footed by Idah’s intervention.
