The Grimsby chairman, Jason Stockwood, had dared supporters to dream as his club attempted to reach the quarter-finals of the FA Cup for the first time since 1939. But not even the most diehard of the more than 4,000 Grimbarians who made the 460-mile round trip to the south coast to see two penalties from Gavan Holohan seal a monumental upset against Southampton could have believed this fairytale story. Accompanied by hundreds of Harry Haddocks – the inflatable mascot that has been present at every big game in the club’s history since they played the holders Wimbledon at this stage of the 1989 competition – the away supporters made it a party atmosphere throughout even after Duje Caleta-Car pulled a goal back midway through the second half. Despite being the lowest-ranked side left in the Cup and with 65 places separating the two clubs in the league pyramid, it was Paul Hurst’s side who prevailed to set up a showdown in the last eight with Brighton after substitute Theo Walcott’s equaliser late on was ruled out for offside by VAR. Southampton had tried to stop visiting supporters from bringing their mascot because Premier League grounds usually have a blanket ban on inflatables. But after suggestions from some Grimsby fans that the top-flight club should have bigger fish to fry given their struggles against relegation this season, the hosts relented and Harry Haddock seemed to be everywhere as Hurst and his players took the applause at the final whistle. The number of empty seats around the rest of the stadium was in marked contrast as Southampton fans perhaps wisely opted to watch on television instead, with the interim manager, Rubén Sellés, having failed to spark a reaction from his players despite making nine changes to the side that lost so tamely to Leeds on Saturday to leave them bottom of the Premier League. National League playoff winners at Wembley last season, Grimsby are struggling down in the lower reaches of League Two but have beaten three League One teams – including promotion-chasing Plymouth 5-1 – before knocking out Luton in the last round to get to this stage. They now have a Premier League scalp to add to their collection after an evening when everything seemed to go their way from the moment that Sékou Mara’s strike was disallowed for offside after 17 minutes. Mislav Orsic then saw his effort curl just past the far post before Mara dribbled a disappointing shot straight at Max Crocombe in the Grimsby goal. But while Southampton dominated possession, it was hard to see from where the breakthrough might come. The answer came at the other end just before the break when a cross from Joshua Emmanuel almost set up Harry Clifton at the back post. Replays showed Lyanco had diverted the cross with his hand so, after much deliberation at the pitchside monitor and to the delight of the visiting fans, the referee, Thomas Bramall, awarded the spot-kick that was coolly dispatched by Holohan. Southampton came out of the blocks quickly in the second half as Moussa Djenepo’s shot was deflected wide but the wind was taken out their sails by a moment of madness from Caleta-Car. The Croatian defender tangled with Danilo Orsi as the Grimsby striker chased down a long ball and appeared to shove him in the back, with Bramall this time showing no hesitation in pointing to the spot. Once again, Holohan made no mistake to send the away fans into raptures. Sellés threw on James Ward-Prowse and within six minutes Southampton had pulled one back when Caleta-Car finished from close range. John McAtee had a chance to seal it when he raced clear on goal before Walcott was denied his equaliser on a dramatic night to cherish for Stockwood and Grimsby’s delirious fans.