Two people died in the firebomb attack.
A fifth person has been arrested in connection with the deaths of a woman and young boy in Offaly last year.
Four year old Tadgh Farrell and his grand-aunt Mary Holt were killed when a house in Edenderry was firebombed last December.
Four people were detained as part of the investigation in recent day, three men and a teenage boy.
Gardai say they've now made a fifth arrest, a man aged in his 20's.
All five are being questioned at Garda stations across the midlands.
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