Shot Irish gangster John Gilligan told gardai "I have to get out of here" before demanding to be taken from his hospital bed to an early-morning ferry.
The injured criminal refused to tell detectives of his next move as he boarded the 2.15am boat to Holyhead yesterday.
The frail 62-year-old is believed to have travelled to Birmingham, where he has some relatives.
Despite checking himself out of hospital it is likely that he will still require significant medical treatment, including physiotherapy for a broken leg he suffered during the attempt on his life on March 1.
His ultimate plan is to make his way to Amsterdam, where he will hide out from his underworld rivals who want him dead.
Sources say Gilligan finally accepted that he was in real danger after his 'minder' Stephen 'Dougie' Moran was murdered in a gun attack on Saturday night.
"Even after he was shot himself Gilligan remained defiant that he was in control of his situation, but he was very shaken by Moran's murder. Within hours he was planning an escape," explained the source.
The former gang boss was escorted by armed detectives to Dublin Port, where he boarded a ferry.
"He had talked about using a wig as a disguise but in the end it was decided that a quick exit was the best option. He was genuinely scared. It was the first time he has shown fear.
"Gilligan had always intended to make a statement to his attackers by walking out of hospital with the same bravado that he has displayed since his release from prison; but after Moran's shooting he couldn't get out quickly enough. He told gardai and his relatives, 'I have to get out of here'," a source said.
Gilligan looked gaunt as he was wheeled from James Connolly Memorial Hospital, where he has been recovering since being shot several times at his brother's home in Clondalkin.
The drug trafficker, who ordered the murder of journalist Veronica Guerin in 1996, was dressed in a black jacket and tracksuit bottoms and was wearing a black flat cap. There was visible bruising on his face where one bullet had grazed his head.
Gilligan, who appears to have lost a considerable amount of weight while in hospital, was collected in a black C-class Mercedes.
Armed gardai followed him as far as the ferry terminal at North Wall, but that is where their remit stopped.
It is estimated that it has cost the State more than €35,000 (£29,286) in accommodation and security for the two weeks he was in the Blanchardstown hospital. Apart from the hospital bill, the costs for Gilligan's Garda security topped more than €21,000 (£17,571).
Gardai had to provide the high-security watch on Gilligan for fear that the assassins who are hell-bent on killing him might be tempted to put other patients, staff and members of the public at risk in another murder attempt.
Two gardai were placed on duty outside his room 24-hours-a-day from Saturday, March 1, with each shift lasting 10 hours. Meanwhile, gardai are still trying to find a definite motive for the murder of Moran at his home in Lucan.
Moran – who was a major target for the Criminal Assets Bureau – had been driving Gilligan around in a bullet-proof 4x4. The main line of investigation remains his links to Gilligan but detectives are also trying to establish if he was rowing with any other criminal elements.
John Gilligan rose to prominence after the murder of the investigative journalist Veronica Guerin. He was acquitted of her murder, but is believed to have ordered it. In 2001 he was sentenced to 28 years in prison for drug dealing. This was later reduced to 20 years on appeal. On October 15, 2013, Gilligan was released after 17 years in prison.