With the heat on John Hughes' recently criticised team, the visiting side produced a big result and performance. McKay, with just one goal in his previous 10 matches, also served a timely reminder of his penalty box prowess with eight minutes left.
It proved a riveting, manically paced confrontation full of full-blooded desire and aggression, as well as ferocious, breathlessly entertaining football.
When the dust settled Inverness had eased pressure on Hughes and kept alive faint European aspirations, while County remained just three points above the relegation play-off spot.
Before kick-off, no fewer than 18 points separated top-six Inverness from their great foes in the bottom half of the table. Yet current form approaching the third and final north showdown of the Premiership season was an entirely different matter.
Hughes' side had managed just one win in nine outings, while County - a much more potent force since New Year - were fresh from impressive draws against Aberdeen and Celtic.
But while County had triumphed 2-1 in Inverness on New Year's Day, Hughes' side had the psychological advantage of having trounced County 3-0 in Dingwall in late February's rescheduled tie.
County, given that compelling recent form, were unchanged for a third game running, but the vistors counted three switches. Central defender Josh Meekings returned from suspension to replace the injured Danny Devine, but midfield pillar Ross Draper was a surprise absentee after a stomach bug.
Aaron Doran made his return for the visitors after missing four matches with a hamstring problem, while Marley Watkins replaced young Liam Polworth.
The pace from the first whistle was always going to be furious, but even by Highland derby standards it was frenetic. Watkins earned a third-minute yellow card for a blatant lunge of the foot as County goalkeeper Mark Brown collected a pass-back. Greg Tansey escaped similar punishment for another late tackle on Graham Carey four minutes later.
The hosts survived their first scare after nine minutes as Brown went to collect a Graeme Shinnie cross swept in from the left. Yann Songo'o, the on-loan Blackburn defender, failed to hear his goalkeeper's shout and flicked a header backwards just wide of his own goal.
County's Richie Brittain threatened from distance and a flicked Melvin De Leeuw header rolled just wide as County pressed. But it was Inverness who went closest to taking the lead after 16 minutes.
Watkins' pass fed Billy Mckay into space and the Inverness top scorer drove low and hard, only to see Brown's outstretched leg divert the ball for a corner.
Wild County celebrations were cut short by the off-side flag after 35 minutes. Graham Carey's free-kick sat up perfectly for Yoann Arquin and the big French striker thumped a header goalward. Cameroonian Songo'o intervened with a volley close-in to find the net, but seemed to have sneaked just beyond the line before the ball was played.
A minute later, Carl Tremarco became the third Inverness player to enter the book with a reckless lunge that sent County midfielder Filip Kiss head-over-heels and then writhing painfully on the ground.
With half-time approaching, Shinnie's stringing strike was parried for a corner by Brown at his right-hand post.
Moments later, Arquin and de Leeuw combined for County before Greek left back Evangelos Oikonomou cracked in a fierce attempt, but straight at Brill.
It looked as if the half would finish goalless, but Hughes' side had other ideas. From a 30-yard free-kick, Tansey struck with phenomenal force. Brown, again, parried but McKay recovered the ball at the edge of the box and fed it back to Doran.
The Irish winger's response from an angle 30 yards out on the left was exquisite as he sent the ball arcing into the far roof of the net.
County could hardly have asked for a better response early in the second period. Melvin De Leeuw's delivery from the left found Songo'o and he attempted an overhead kick at the edge of the box but took an air swipe.
The ball fell to Kiss and the on-loan Cardiff City man executed the chance brilliantly, angling a right-foot shot into the net from 10 yards.
From the next attack, Inverness had the home support sweating as David Raven's ambitious dig was just touched over the bar by a desperately back-tracking Brown. Hughes' men had a strong penalty claim after 57 minutes when Kiss appeared to trip Watkins, with his back to goal.
As the home pressure continued, there was fury as Shinnie was penalised outside the box for landing a foot in Kiss's face. Television replays later confirmed that the incident had taken place inside the area.
Chaotic, end-to-end action continued and culminated in Mckay's dramatic return to scoring form.
Watkins' ball in from the left was controlled superbly by the Northern Ireland internationalist who scored at the second attempt. He cracked the crossbar before the match was done, but it mattered not.