The striker scored twice in the first half, sandwiching an equaliser from Tractor Boys defender Damien Delaney, who was sent off in the 37th minute for a professional foul on Holt.
Roy Keane's men rarely threatened an equaliser in the second half and, after Holt completed his treble in the 76th minute, substitute Wes Hoolahan added a fourth two minutes later.
Holt dovetailed effectively with Chris Martin and Lambert enthused: "I thought Grant was unplayable at times.
"It was a superb performance. He was always harassing people and not letting them settle and he stuck his chances away too.
"He is never going to be a striker with great finesse but when he's playing like a typical old-fashioned number nine he is brilliant."
Source: PA
Source: PA