Celtic weren’t to be denied for long and had a third in the 66th minute from a stunning free-kick from Armstrong. Fon Williams was caught napping as the Scottish midfielder curled in a powerful effort from the left-edge of the box that beat the ‘keeper at his near post.
Dembele, who grabbed a hat-trick in the 6-0 Scottish Cup win over Inverness just 18 days ago, helped himself to his second of the match in the 73rd minute.
The French under-21 international beat the off-side trap to race on to Kieran Tierney’s lofted pass and lift the ball over the out-rushing Fon William for his 11th goal in his past five matches.
Inverness manager Richie Foran gave a damning indictment of his side’s performance.
“We lacked courage and belief. We just didn’t turn up tonight and we probably lost the game before we even started for some players,” Foran said.
“I’m not even sure Celtic got out of second gear.”
Elsewhere, Ross County defeated Hearts 1-0 at Tynecastle that leaves Jambos’ boss Ian Cathro with just four wins in 15 matches.