The five, four men and a woman, aged between 20 and 36, were arrested in raids carried out in the Ballinacurra Weston area of Limerick and brought to Garda stations in Ennis, Shannon and Killaloe.
The arrests were made as part of the investigation into the death of Richard “Happy” Kelly, aged 17.
He was last seen alive on April 24, 2006.
His body was found in Lough Brigid, near O’Callaghan’s Mills, last November after it became entangled in a fisherman’s nets.
His body had been weighed down by blocks.
It is suspected that a Southill drugs gang murdered Richard Kelly because he took a car in which they had stashed drugs, guns and money.
The dead youth was a known joyrider.