Sources say that officers had devised measures to deal with a potential outbreak of violence if the 150-strong congregation arrived from the North.
Gardai established that there was a "credible threat" to the protestors from dissident republicans, including associates of the murdered terror boss Alan Ryan.
Loyalist representatives were told by detectives that their lives would be under threat if they went ahead with the demonstration on Saturday.
As part of their contingency plans, gardai were preparing to deploy the armed Special Detective unit, as well as the riot squad and undercover officers.
The loyalists were also informed that the Tricolour does not fly above Leinster House on Saturdays because the Dail is not in session.
Willie Frazer, chairman of the Families Acting for Innocent Relatives (FAIR), confirmed that the event was shelved following discussions with gardai.
"I am pleased to inform you all that we are now arranging a full meeting with the Garda Siochana in Dublin in the near future," Mr Frazer said.
"After very constructive communications with the Garda Siochana, I can confirm that we will now be travelling down on a date in the near future to discuss our many concerns around IRA collusion and many other issues," he added.
While loyalist protestors have claimed that the event in Dublin has only been put "on hold", sources are sceptical that it will take place at all.
The news of the cancellation sparked several taunts by dissidents republicans.
In a statement online, the Dublin branch of the 'County Sovereign Movement' stated:
"No doubt republicans will confront these... loyalist bigots not on their flags issue, but on the continued sectarian thinking that causes these people to attack nationalist areas and support for the occupational forces who occupy the north."
The news of the cancellation has been widely welcomed by politicians and city retailers.
Fianna Fail Justice spokesman Niall Collins dubbed the plans "ill-judged".
"I am pleased that Mr Frazer appears to have heeded my calls and calls from others to think again about this protest. It was ill-judged and had potentially serious consequences.
"It will come as a relief to many people that it is now unlikely to go ahead this weekend, not least Dublin city centre's retailers," he said.
Deputy Collins also urged Mr Frazer to abandon any plans he may have to stage a demonstration in the future.
Mr Frazer had maintained that the demonstration was to take place in order to highlight Belfast City Council's decision to restrict the days the Union flag is flown at city hall.
Sinn Fein leader Gerry Adams called on any republicans opposed to the loyalist plans to "ignore" them.
"If Willie Frazer, and indeed anyone else, wants to come and protest in any part of this state, we should ignore it.
"No one should raise a voice or throw a stone or counter-protest.
"Let them come let them have their protest and let them go," he added.