He lost most of his memories when he took a lightsaber through the head and clinically died. He'd had a chip implanted in his brain when he was little that allowed him better mental control than somone of the same age normally would. When he 'died' his mother put his body in stasis and with the help of medical droids implanted increasingly sophisticated augmentation devices to assist in taking over the lost sections of the brain. Unfortunately Irek came out of stasis early due to groundquake damage from the Vong attack on Coruscant. He killed his mother and escaped the facility to wreak havoc on the city. When he escaped he was missing almost all of the special programming that would have been transferred into the expanded implants. Artificial memory, personality algorithims, light saber techniques, communication, higher level judgement. And his physical attributes wern't quite up to what the 'finished' stage would have been. Despite all of this he was still incredibly powerful. EDIT: I did it again didn't I? I killed a thread.