High profile republican Colin Duffy, 44, and a second man aged 31 have been released unconditionally, a spokeswoman for the PSNI said.
The pair were detained in Lurgan, Co Armagh, miles from where David Black was ambushed on the M1 motorway on his way to work at the top security prison near Lisburn, Co Antrim.
A third man arrested by Gardai is still being questioned in Co Leitrim.
Duffy has been cleared of murder charges on three previous occasions - the latest last January after two soldiers were shot dead outside Massereene Army barracks.
He was arrested on Friday at his home in the Kilwilkie estate, where republicans opposed to the peace process have huge support.
Three men, aged 31, 29 and 44, have been arrested following the killing of prison officer David Black in Northern Ireland.
Leaders in Northern Ireland have condemned the "horrific" motorway murder of a veteran prison officer by suspected dissident republicans.