Both criminals are serving lengthy jail sentences for the massive drugs bust which caused Marlo Hyland's prolific drugs gang to implode in 2006 and which ultimately led to Hyland's murder.
Hynes (46), of Park Close, Dunshaughlin, Co Meath, and Mangan (44), of Whitestown Green, Blanchardstown, were snared after a massive operation by the Garda National Drugs Unit at Browns Barn, south Co Dublin, on July 31, 2006.
More than €2m worth of cannabis resin was seized by gardai in 14 boxes split between a Ford Transit van and an Opel Astra car in Browns Barn public house car park. The two men were arrested nearby.
After a dramatic trial in November 2009, Judge Patrick McCartan imposed a 12-year sentence on Hynes and a 14-year sentence on Mangan.
A senior source said: "This bust led to huge paranoia in Hyland's gang. It was just one of many operations against his crew as part of Operation Oak but it was the final straw for some of his key lieutenants who were convinced that Hyland was ratting on them to gardai."
Operation Oak was set up in September 2005 and was one of the most successful garda operations in recent history.
By the time that Marlo was murdered -- 15 months after the operation began -- gardai had seized 30kgs of heroin, with a street value of €8m; 35kgs of cocaine, worth €2.5m; 1.4 tonnes of cannabis, worth €10m; as well as four stolen vehicles, firearms, ammunition and cash.
It also led to 41 arrests and 26 suspects being brought before the courts on charges ranging from possession of drugs with intent to sell or supply, robbery and possession of firearms.
When the Browns Barn bust happened in 2006 both Hynes and Mangan were on bail for other serious offences linked to Marlo's crew at the time of the operation.
Only five months before that bust, Mangan was arrested in Drumconda, as part of Operation Oak, and was caught with more than €300,000 worth of cannabis. When he was arrested on that occasion in February 2006, asked what was in the bag, he replied: "Hash, what do you think? That's what happens when you owe people money." He said "the job" was worth €2,000 to him off a €9,000 debt.
Just eight months after 'Marlo' and Anthony Campbell were shot dead, Mangan was arrested with a loaded Luger pistol down his trousers in the Comet pub in Santry in August, 2007.
He was on bail on two charges of possession of cannabis at this time. Mangan told gardai his life had been threatened between eight and 10 times that year and he was only carrying the pistol for his safety.
Mangan is serving 10 years on top of the 14-year sentence imposed on him in 2009 for these offences.
And William Hynes was known to be involved in large-scale drug dealing ever since he linked up with Hyland in the mid-1990s. Hynes was given a five-year jail sentence in February 2000, after he was caught red handed with more than IR£2m worth of cannabis owned by the gang in Co Meath, in April, 1998.
After his release from prison in 2003, Hynes continued to be involved in serious criminality and made his way up the gang's pecking order.