Adrian Crevan Mackin, responsible for the murder of Garda Tony Golden, had a string of previous convictions including possession of a handgun, IRA membership and possession of bestial pornography.
Mackin shot dead Garda Golden and attacked his partner, Siobhan Phillips, before turning the gun on himself in Omeath in County Louth on 11 Sunday October.
Garda Golden had been assisting Ms Phillips to retrieve her things at her home when the shootings took place. She had been the victim of domestic violence.
Details emerging this evening show Mackin, 25, was convicted of possession of a loaded automatic handgun in Newry in December 2013.
He was given a three year sentence, suspended for three years.
Soon after his conviction, he moved over the border to Omeath in County Louth.
RTÉ 's Prime Time programme has revealed that when he was charged with membership of the IRA in January 2015 at the Special Criminal Court in Dublin, he was only 13 months into the three year suspended sentence imposed in the north.
The Director of Public Prosecutions consented to bail and gardaí did not oppose bail on the IRA charge.
It is not known if the authorities were aware of his conviction for the serious offence of possessing a loaded automatic handgun in Northern Ireland.
Neither gardaí nor the DPP would comment on the specific case.
Mackin was convicted of other charges, including bestial pornography, criminal damage and shoplifting, but he was not serving sentences for these offences at the time he committed the murder of Garda Tony Golden.