Specialist gardai were forced to act quickly after the discovery of the note, which gave specific details of the assassination attempt, including where and at what time to kill the man in south Dublin.
Sources say that detectives informed the solicitor of the threat against him and moved him out of his home in Rathmines.
He was brought to a secure location for several days.
After foiling the hit, detectives waited a number of months before arresting the gangster in the Dublin jail last week.
Gardai kicked in the door of his jail cell and arrested him.
The gangland criminal was questioned for 18 hours at a Dublin station about the plot before being transferred back to prison.
Details of last week's arrest operation have not been revealed before today and sources say that gardai are confident of pressing charges in the case.
A source said: "You could be looking at conspiracy to murder and threats to kill charges in this case.
"He decided to take him out and drew up a detailed plan to this effect, but was snared when gardai discovered the hand-written note," said a source.
"He then tried to call off the hit after gardai seized the note," the source added.
Information gleaned from the note states that John Wilson and another criminal were to be involved in the gun murder.
Dad-of-two John Wilson (35) was shot dead at his Ballyfermot home last September -- just months before gardai foiled the sinister plot he had been enlisted in.
John's nephew Luke (18) was lucky to escape when he was shot in the head at the National Memorial Park near Inchicore earlier this month.
Meanwhile, the foiled Rathmines gun attack is understood to be just one of over a dozen violent incidents the jailed gangster has ordered against his former associates and their families, who he has accused of "ratting him up."
None of these incidents can be outlined here for legal reasons.