Controversial website Ask.fm has supplied the IP addresses which were used to post vile comments before the girls' deaths.
Ciara (15), from Co Leitrim, took her own life in September last year. Erin (13), from Donegal, died five weeks later.
The Herald has now learned that Ask.fm has traced five IP addresses used to send cruel messages to Erin, and at least three in Ciara's case.
The girls' deaths are completely unrelated but both were linked to bullying on the Latvian-based social media site ask.fm which allows members to post comments anonymously online.
Sources say that the website has "cooperated fully" with gardai who are preparing files for separate coroners' inquests.
Officers are now preparing to interview all those using the IP addresses.
"The technology and the know-how needed is fairly straight forward and a number of people will be interviewed and statements will be taken before the inquests," one source told the Herald.
In the Donegal case three schoolgirls wrongly accused of the online bullying of Erin are anxious for the real culprits to be tracked down so they can clear their names.
Gardai believe the real trolls posed as them to torment Erin.
The innocent girls' homes have been attacked and two of them have been verbally abused.
Ironically they have also become victims of cyberbullies who posted death threats online against the girls. "It has been a nightmare for them," said one friend of the families.
"They've only ever wanted Ask.fm to release the details of those who posted comments to Erin so that the girls can finally clear their names."
The bullying of the girls began again last week after the death of Hannah Smith (14) in Leicestershire.
Ask.fm has released details of IP addresses used to bully her to English police. The Donegal families visited gardai last year after Erin's death, anxious to clear their children's names.
All gave statements saying their children had been removed from the Ask.fm site months before Erin's suicide.
The Latvian-based site said that it had been cooperating with the Irish authorities investigating the deaths of Ciara and Erin.
"This included passing on details of the IP addresses used for making comments," said a spokesman.
The site repeated that although it is possible to post anonymously to the site, that in almost all cases it is possible for Ask.fm to identify users. They said they worked to ensure this information is accessible to the appropriate legal authorities.
The company has been left reeling from the withdrawal of major advertisers in the wake of the death of Hannah Smith.
It is currently carrying out a review of its operations and will publish the results of those tomorrow.
Earlier this week Erin's mother Lorraine told Channel 4 News in the UK that her daughter had named the site in her suicide note.
A Garda spokeswoman said investigations into the two Irish deaths are continuing.
"The investigation for the coroner's court in relation to both deaths are ongoing and therefore we are not in a position to make any comments," she added.