Tail-ender Kirby, 30, was hospitalised with suspected concussion after being struck behind the left ear attempting to evade a short ball from Luke Wright during the afternoon session.
He was left unconscious for half a minute after collapsing in a heap and passed out again in the dressing room having been led from the field.
He initially came round after Sussex captain Chris Adams and wicketkeeper Matt Prior used their first-aid knowledge and moved him into the recovery position.
The rest of Sussex"s fielders, meanwhile, signalled frantically to the dressing rooms for medical aid and England and Wales Cricket Board physio Steve McCaig was soon on the scene.
“Matt and Grizz (Adams) went in, held his neck, which is obviously a key thing, lifted him into the safety position first and then Matt just checked to see if his tongue was there," revealed batsman Chris Nash.
“That is the one thing we have heard about previously when people have been hit on the head. It is a horrible thing - you never want to see anyone hit like that."
Although Gloucestershire fast bowler Kirby, who took four wickets in the first innings, laughed and joked with his opponents while receiving treatment and wanted to carry on, he was 'retired hurt" by McCaig - closing the MCC innings on 205.
“When he got up he was as good as gold," Nash added. “He"s a really great character of the game.
“He cracked a joke and everyone laughed, which helped relax things a little bit. He wanted to carry on but when he got up he was a bit dizzy."
Minutes later he passed out for a second time and left the ground in an ambulance, accompanied by ECB bowling coach Kevin Shine, bound for medical observation at The Royal Free Hospital, Hampstead, where he was detained overnight.
By the close, county champions Sussex had progressed to 174 for two in their second innings, an overall advantage of 140.