Replacements: Warren Larkin, Alan Matthews, Rhodri Jones, Kevin Quinlan, Scott Deasy.
Replacements: Rory Daly, Chris Corkery, Yasin Browne, Chris Quinn, Alan Ross.
Cork Con had started the game in fine fashion, building from the halfway line for impressive back rower James Ryan to score an eighth minute try which Lyons - their captain in the absence of Munster-tied Gerry Hurley - converted.
Prop Gavin Duffy provided the try-scoring pass as Ryan finished off a good team effort, with the visitors evidently eager to end a run of three successive losses.
However, that opening score was cancelled out by Lansdowne winger Foster Horan eight minutes later, the youngster stepping away from a couple of defenders for a terrific individual try.
Tom Daly converted and added two quick penalties for a 13-7 Lansdowne lead, but they lost their skipper Willie Earle and tighthead prop Adam Boland to the sin-bin before half-time.
Constitution forced the issue in the scrum, with current Ireland Under-20 hooker Max Abbott showing well, and they registered a penalty try from a five-metre scrum to take a 14-13 advantage into the break.
Lansdowne exerted the early pressure on the resumption and it eventually paid off when Charlie Butterworth barged over in the right corner in the 57th minute, with support from his back row colleague Clive Ross.
However, Cork Con had the better of the final quarter and centre Michael Keyes' came up with the crucial play, timing his pass to perfection to release Quinn for the game's decisive try.
Lyons converted and then tagged on the insurance score - a late penalty - as Constitution leapfrogged Lansdowne into third place in the Division 1A table.
It was a memorable day out for the Leesiders as they played on the hallowed turf for the first time since their AIB Cup final victory over St. Mary's College back in 2006.
Cork Con head coach Tom Mulcahy told the Irish Examiner: "I thought we played very good accurate rugby. At times they had control and when they did they scored, but our guys stuck to it.
"It was a difficult thing coming up here off the back of three losses. To score four tries against Belvedere (last time) and lose that game was heartbreaking.
"We were just trying to get back on the road. We got the result and no better place to get it than against the champions."
Giving his assessment, Lansdowne boss Mike Ruddock admitted: "It was a tough game and there was a crucial spell when we had two yellow cards and a penalty try against us.
"We did better for 30 minutes in the second half but then missed one tackle and conceded a try and that was the end of it really. We were on top for most of the second half but let our guard down once and they punished us."
Referee: Stuart Gaffikin (IRFU)