The match-up, in Froch's home city, was the second in the Super Six super-middleweight tournament masterminded by American broadcaster Showtime.
Earlier, Germany's Arthur Abraham knocked out American Jermain Taylor with 14 seconds remaining in the 12th round, just as Froch had done in April.
"I'm a little bit dissatisfied but I can only beat what's in front of me," Froch told BBC Radio 5 live.
"We never really stood and had a trade and got going with anything," added Froch, who is now undefeated in 26 fights with 20 knockouts.
Dirrell believed he had done enough to win the fight.
"I thought I held him off enough, boxed him enough to get a decision," said Dirrell, who falls to 18 wins with 13 knockouts and one defeat.
"We know where we're at, but I'm going to hold my head with pride... I still don't know why (the referee) took the point from me.
Froch: "I'm a little bit dissatisfied but I can only beat what's in front of me"
"I'm still clueless on the point for leaning on him. He'd been holding me, hitting me in the back of the head the whole time, bringing me down on one knee. He'd been rough the whole fight."
Eight-thousand fans crammed into the Trent FM Arena to see Froch defend his title for the second time, and they were forced to wait until 0245 BST for the action to get under way, for the benefit of the American television audience.
The reward for their patience was an ugly affair, with clean punches at a premium, dubious tactics aplenty and Dirrell falling to the canvas at regular intervals.
After a cagey opener, the 26-year-old Dirrell served notice of his slick skills in round two, landing with a couple of sharp left-hand counters and two stiff left jabs as he switched between a southpaw and orthodox stance.
Froch landed with his first meaningful shot in round three, a solid right cross, but with Dirrell constantly switching, holding on the inside or pirouetting out of trouble, all the signs were that the champion was in for an exasperating time.
Froch, 32, betrayed his irritation in round five, throwing Dirrell to the floor, and Froch followed up with two chopping rights on the inside in round six.
Bizarrely, Dirrell, who had looked serene in the first half of the fight and displayed some admirable defensive skills, started to complain in round seven, first about Froch's use of his forearm, and then about hitting behind the head.
With Dirrell spending so much time holding round the waist, the referee was right to ignore his pleas, and Froch appeared buoyed by his rival's distress signals, wobbling Dirrell with a big right hand in round eight.
Dirrell complained some more in round nine, but it was he who was deducted a point in round 10 after grabbing Froch and marching him towards the ropes.
The docked point spurred Dirrell into action and he had Froch in trouble with a huge left hand down the pipe later in the round, but he was unable to capitalise.
With Dirrell opening his shoulders and landing at will with left hands in rounds 11 and 12, it left observers wondering why he had been so cautious for so long.
And while he felt he had done enough to win the fight, ultimately it was his defensive mindset that proved his downfall, with two of the judges preferring Froch's marginally more confrontational style.
The win means Froch takes two points into his next Super Six encounter with the tournament favourite, Denmark's Mikkel Kessler.
Kessler, who was beaten by former super-middleweight king Joe Calzaghe in 2007, faces American Andre Ward next month. Abraham, who fights Dirrell next, earned three points for his knockout victory over Taylor.
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