Celtic moved to within 10 points of guaranteeing a seventh successive Premiership title with a comfortable victory over bottom side Ross County.
County striker Billy Mckay had a good early chance saved before Moussa Dembele converted a penalty after Jason Naismith brought down Scott Sinclair.
The visitors’ hopes evaporated when captain Andrew Davies was sent off.
The impressive Armstrong drilled in the second before a lovely finish from Tom Rogic sealed a straightforward victory.
Three more wins from their remaining seven games – given their vastly superior goal difference – would effectively seal part five of a potential ‘double treble’ for Brendan Rodgers’ side.
They now have a 12-point lead over Rangers – who have played a game more – and Aberdeen.
County, who have won just one of their past 18 Premiership matches, remain three points adrift of Partick Thistle, who lost 2-0 at Hibernian.
Celtic immediately looked menacing, but in between Armstrong shooting over and Dembele dragging his effort wide, the visitors gave their hosts a real scare.
Mckay, a previously potent presence who has struggled for goals this season, was played in neatly and it needed an alert stop by Celtic goalkeeper Scott Bain to prevent a goal.
Armstrong found pockets of space all over the pitch and was at the heart of Celtic’s attacking play again when his rehearsed, low corner kick set up Sinclair for a low shot that was deflected fractionally wide.
There was no argument from Naismith when Sinclair’s ‘Cruyff turn’ near the byeline drew a foul from the defender and referee Steven McLean pointed to the spot after 25 minutes.
Dembele stepped up to slot the slot the ball into the bottom right-hand corner for his 14th goal of the season.
Any slim chance the Dingwall side had of getting anything from the game ended six minutes before the interval when captain Davies was dismissed for an apparent lunging stamp on Celtic skipper Scott Brown.
Stuart Kettlewell’s men were dealt another blow soon afterward as goalkeeper Aaron McCarey had to be replaced because of injury – Scott Fox entering the fray in the closing minutes of the first half.
Armstrong’s energy and direct approach deserved a goal, and he got his fifth of the season for Celtic early in the second half.
The Scotland midfielder had time to aim a perfectly drilled shot into the bottom corner from the edge of the box after James Forrest teed him up.
Forrest earned his second assist of the day on the hour playing in Rogic to score Celtic’s third in a classy manner. The Australian gathered the ball beautifully on his left foot and showed equally good skill to plant a right-footed finish low past Fox.
Striker Leigh Griffiths made his return to competitive action for the first time since January and was given 20 minutes to get up to match speed.
But it was County who almost finished with a flourish – Ryan Dow nearly netting at the near post following good work by Michael Gardyne.
Celtic manager Brendan Rodgers: “I thought it was a job well done. We started well and didn’t get the goals we deserved but our drive and intensity was good and once we got in front, and their captain was sent off for a really poor challenge, we found that bit of ruthlessness we needed.
“It is another three points to where we want to go.
“Stuart Armstrong had really good intensity in his game. He has been out for a long time and is still finding his fitness, but we gave him 90 minutes and he was really good.
“He ran out of steam a bit towards the end but I am very pleased for him. He had a real goal threat and you saw his energy.”
County interim co-manager Stuart Kettlewell: “A few things went against us – Andrew Davies being sent off, a penalty against us and we lost our goalkeeper to injury.
“We knew it would be a mammoth task in the second half. We took a lot from it as a lot of teams can capitulate and fold at that point.
“We felt the players showed a real degree of heart today, which is massive. This was never going to define our season.
“I’ve not seen the sending-off incident. Andrew is an honest guy, he is our leader on the pitch and is disappointed he has been sent off today. We will look at the incident and take it from there.”