The Belfast native defeated Paddy Donovan in a controversial split decision, to win the prestigious belt, despite going into the bout as a heavy underdog.
In their first event in March, the Limerick man dominated the fight, comfortably beating Crocker by points before he was disqualified for punching after the bell.
Pundits, fans and so-called boxing experts expected the southern star to cruise to victory in the rematch at Windsor Park on Saturday night, but the Ulsterman dropped his opponent twice, giving him the edge on the judges’ scorecards in the end.
“First of all I want to thank God,” said an emotional Crocker afterwards.
“I have prayed every day for years to get this decision, and nobody gave me a chance going into this fight.
“I just want to say to everyone who was giving me