Officers made the breakthrough while investigating a terrifying cash-in-transit robbery in Tallaght. A source said: "This is being viewed as a great result by gardai -- two of the four individuals in custody today have been on the garda radar for a very long time indeed."
The men targeted a delivery van at at Jobstown Post Office shortly after midday yesterday.
One of the raiders, armed with a 9mm handgun, threatened a male security guard and put the weapon to his head. He was ordered to hand over the cash box, which contained €50,000, or he would be shot.
The gunman grabbed the box and was then collected by his accomplices in a stolen car. But their cash haul was destroyed by a red dye when the gangsters attempted to open the cash box.
Officers from Tallaght were quickly on the scene and a short time later recovered the getaway car, which had been abandoned off the Kiltipper Road.
The gun used in the robbery was found inside the vehicle.
Armed detectives then raided a house at the Cloonmore Green area and found two suspects in the bathroom where the money, covered in red dye, had been submerged in water in an attempt to wash the dye away.
Two other people, a man and a woman who were also in the house, were arrested but are not thought to be suspects.
The two suspects, who are thought to have played a major role in the cash-in-transit robbery, are a Traveller from the Tallaght area who is part of a feared robbery gang and a notorious young gangland figure from the Ballyfermot area.
The Ballyfermot hood, aged in his early 20s, was closely linked to murdered Tallaght drug dealer Stephen O' Halloran (20), who was shot dead in January, 2009.
His crew have been involved in a number of violent incidents, including the murder of Kevin Kenny (32) who was stabbed to death outside a Ballyfermot pub in August, 2011 as well as dealing in crack cocaine and crystal meth.
They are also suspected of shooting up houses and armed robberies in the Ballyfermot, Clondalkin and Tallaght areas. Gardai are confident they will get charges against at least two of the suspects in the case.