McGregor was in the house at the O2 Arena as his alcoholic beverage sponsored the show.
And he was joyous after AJ knocked Helenius out in round seven with a hellacious right hand.
Joshua, 33, immediately fled the ring and took a lap around in celebration, bumping into McGregor, 34.
And with a pint in his hand the Irishman fed AJ – who sported a bloodied nose – a sip of the creamy stout.
McGregor stole the show on the night as he called out YouTuber-turned boxer KSI – who was also ringside.
He said: “Big congrats to AJ, big congrats to Eddie, big congrats Forged Irish Stout.
“And I know your man KSI is in the crowd and he couldn’t box eggs if he worked in an egg boxing factory.
“So if he wants to get in here I’ll give him a final yoke, bare-knuckle if you want.”
Responding, KSI, 30, tweeted: “Lmao just seen this.
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“Jake Paul must be crying that I’ve got Conor on my d*** already and I didn’t even have to fight an mma fighter.”
KSI returns on October 14 in Manchester against Tommy Fury, 24, who in February beat his arch rival Paul, 26.
Meanwhile, McGregor claimed his UFC comeback bout is set for December against Michael Chandler, 37.