Garda Commissioner Nóirín O’Sullivan said the seizure will make “big inroads into dismantling” the Kinahan organised crime gang.
Sources said the ledger is “extensive” and is suspected of containing cash transactions and payments from individuals.
It is not clear if the information is coded and work is underway to analyse the document.
“The real value of the seizure is what is in that ledger,” said a garda source.
Two men, described as “relative unknowns” in the crime world, were arrested in the operation, conducted by the Garda Drugs and Organised Crime Bureau. The DOCB is concerned at the trend and follows the arrest last month of a man, described as being “way below the radar”, in connection with a major drug and gun haul in north Dublin.
Following a surveillance operation, the DOCB raided an apartment in the Beacon South Quarter complex in Sandyford on Thursday evening.
They arrested a man, aged 42, as he was carrying a bundle of cash into the apartment.
The DOCB suspect the apartment, rented under a fictitious name, was being used as a cash counting centre by the cartel.
A second man, a 32-year-old brother of the first man, was arrested in a follow-up search in Clondalkin.
The two men are being detained at Blackrock Garda Station on suspicion of money laundering offences.
Officers seized more than €300,000 in cash, as well as cash counting machines, a vacuum packing machine, and an array of documentation, including the ledger.
Commissioner O’Sullivan said: “I think it’s really good evidence of the ongoing work to target organised crime: Two people arrested, a very significant seizure of cash, indeed, significant documentation that will make big inroads into dismantling the Kinahan crime gang.”
She said the creation of the DOCB, merging two previous units, and the establishment of the Special Crime Task Force (SCTF) and the Armed Support Unit had made “significant inroads” into organised crime.
“Let it be the Kinahans or the Hutch crime gang, or any other gang, that decides to engage in criminality, we will make sure we address that and pursue it,” the commissioner said.
It is the latest blow against the Kinahan cartel. In April, the DCOB seized 20kg of cocaine, with a street value of €1.4m, and components for semi-automatic weapons in north Dublin. In March, the SCTF seized a submachine gun and drugs in Co Meath.
In operations last January, gardaí seized an assault rifle and 30kg of drugs in Sallins, Co Kildare; 160kg of cannabis herb (street value: €3.2m) in Co Meath; a submachine gun, an assault rifle and silencers in west Dublin, and 1.8 tonnes of cannabis herb (street value: €37m) at Dublin Port. In November, gardaí seized two MP9 submachine guns, with silencers, and three loaded handguns in Cabra, north Dublin.