Five people are now being questioned as part of the investigation into the murders of a 4-year-old boy and his grand aunt in Edenderry, Co Offaly last year.
Gardaí said a fifth arrest, a man in his 20s, was made yesterday evening.
Tadgh Farrell and 60-year-old Mary Holt died in a fire at a house in Castleview Park in Edenderry, after a petrol bomb was thrown through the front window on 6 December.
Tadgh was visiting the house at the time of the attack.
His remains and Ms Holt's remains were discovered in the front room, where the fire was deliberately started.
Tadgh's grandmother, Pauline Holt, was also in the house and was seriously injured in the fire.
Gardaí made several arrests in the investigation on Tuesday.
Two men in their 20s, one in his 30s and a teenage boy were detained.
All five arrested are all being held at garda stations in the east and midlands under Section 5 of the Criminal Justice Act.
They can be questioned for up to seven days.
