They preyed on youngsters both in the UK and abroad, grooming children in Holland and Spain, where they sexually abused a boy.
Police described the pair, who were lovers, as "very, very dangerous, predatory paedophiles" who were "like-minded individuals". The pair also sometimes claimed they were cousins to suit their "own depraved ends", officers said.
Their convictions date back to 1998 when they were jailed for abusing youngsters at their Clyde coast den after luring them with drink and drugs.
O'Neill, 47, was sentenced to eight years at the High Court in Glasgow for sexual abuse charges, with three years imposed for supplying cannabis to the boys which ran concurrently.
Lauchlan, 33, was jailed for six years for sex charges and three years, concurrently, for drugs supply.
The court was told at the time that a series of assaults on boys aged nine to 17 were committed during a six-year period in Ayrshire.
The crimes came to light when one of their victims broke down and told his aunt after he watched a news report about a boy who had been murdered by a paedophile.
And last month they were convicted of more abuse charges against young boys at the High Court in Glasgow.
O'Neill was found guilty of sexually assaulting a 14-year-old boy after drugging him in 2003.
Both men were found guilty of sexually abusing a 14-year-old boy, locking him in a wardrobe and urinating on him in a motor home in Spain in 2004 and intentionally meeting a six-year-old boy intending to engage in unlawful sexual activity with him, between December 2007 and March 2008.
The conviction could not be reported until now due to reporting restrictions on the case.
Police had begun investigating the pair again after mother-of-three Allison McGarrigle vanished in 1997. She was reported missing seven months later.
Officers said the lovers had been on their radar for "some time", and when Mrs McGarrigle went missing they were "obviously of interest". The pair travelled to Holland and Spain where they continued to groom children, detectives said.
They returned to the UK in 2007, settling in Lancashire and informing police of their whereabouts.
Detective Superintendent John Mitchell from Strathclyde Police said: "Lauchlan and O'Neill have, over the years, been predatory paedophiles.
"They have preyed on the most vulnerable people in society and have left a number of victims in their wake.
"I hope that the McGarrigle family can now move on to a more settled and normal life."
Yesterday the pair were found guilty of murder and jailed for life.
The murder trial at the High Court in Glasgow heard Lauchlan and O'Neill were friends of Mrs McGarrigle before her disappearance in June 1997.
The 39-year-old's body has never been found.
Her husband, Robert McGarrigle, told the court his wife did not own a passport and could not drive. He also said she struggled with reading and writing and needed help.