Ronnie Dunbar has been sentenced to life imprisonment for the manslaughter of Melissa Mahon in September 2006.
In May, Dunbar, also known as Ronald McManus, was found not guilty of murder but guilty of the manslaughter of the 14-year-old.
Dunbar showed no emotion as he stood to hear Mr Justice Barry White pass sentence.
Justice White said Ronnie Dunbar, of Rathbraughan Park in Sligo, had preyed upon the teenager's vulnerability and fragility.
He said the 44-year-old disposed of her body in a manner not befitting an animal after taking her life. 'It was not done in panic, but a calculated and deliberate manner.'
Because her remains - which were dumped in the river Bonnet in Sligo - were not found for a year and half, the cause of death could not be properly established.
This, he said, would have been in the jury's mind when it convicted him of manslaughter and not murder.
Justice White said throughout the trial, Dunbar was disdainful, scornful and bordering on contemptuous of his surroundings.
He said there were no mitigating factors and that Dunbar had never apologised or shown any remorse.