Fears are growing of an escalation in the Drogheda feud after part of a highly-sensitive Garda document fell into criminal hands.
Gangsters on one side of the feud have obtained confidential personal information on some of their rivals.
The partial document contains details on lower tier members of one faction and female associates of some of the leading members.
While it is not believed that the leak was deliberate, a major probe is now under way to determine how the information ended up in the wrong hands.
The document contains personal information including photographs of five people allegedly involved in the Drogheda feud.
One of those is a woman with a north Co Dublin address. The other four people in the file are men with addresses in Drogheda.
"This is a serious matter and it is causing a headache for gardai, it is the subject of a major internal investigation," a senior source said.
"At this stage there is nothing to suggest that there was any malicious intent in how this got out or any suggestion of criminal wrongdoing," the senior source added.
It is unclear whether gardai have to provide extra security for the people who feature in the file or whether there is a threat on their lives as a result of the leak.
It is also not known where the photo of the leaked file was taken.
The document has been circulated on social media but not widely, according to sources.
A number of leaks of confidential Garda information have occurred over the years.
Last March a major investigation was launched after a restricted Garda bulletin board of major criminals was leaked from a Dublin station.
A photo of the notice board, which also included images of an Islamic extremist banned from a mosque, sex offenders and members of a west Dublin crime gang, was taken in a Dublin station.
It is understood that an investigation established that no garda was responsible for the leak on that occasion, and an outside civilian contractor was responsible.
In 2008 members of the Brian Rattigan gang gained access to a secret Garda file which contained information about their deadly rivals in the 'Fat' Freddie Thompson mob during the height of the Crumlin/Drimnagh feud that claimed over 15 lives.