It is understood three of those being held were arrested in prisons. Two others were arrested in Limerick.
They are being questioned about the murders of rugby player Shane Geoghegan, 28, Roy Collins, 34, and 20-year-old James Cronin.
They are also being questioning about other criminal activity, including a number of shootings.
Follow-up searches are continuing.
Garryowen rugby player Mr Geoghegan was shot dead near his home in Dooradoyle in November 2008; Mr Collins was shot as he worked in the family amusement arcade in Roxboro and Mr Cronin's body was found buried in undergrowth in a field near the railway line in Limerick city centre.