It is understood that a petrol bomb was thrown at the front door of the home in the Birchwood area of Waterford City at around 5am on Tuesday.
The deadly blaze took the lives of nine-year-old Jayden English, his brother Callum, 12, and their parents Philip and Kim.
A 14-year-old girl and a six-year-old boy remain in critical condition at Children’s Hospital Ireland (CHI) Crumlin.
Jayden and his younger brother previously featured on the BUMBLEance social media page. BUMBLEance is Ireland’s dedicated children’s ambulance service.
A picture shared on the service’s Facebook page in 2021 showed Jayden smiling for the camera as Peppa Pig played on the TV beside him.
"#Kingbee Jayden watching Peppa Pig and enjoying his trip to CMRF Crumlin onboard the #BUMBLEance," the caption read.
The previous year, the service revealed that both Jayden and his brother were born with heart defects and shared the very same "chromosome disorder."
Waterford-based paramedic Áine Phelan had worked with the family during her time in the service.
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RTÉ reports that the paramedic said it had been an "absolute privilege" to travel alongside Jayden at the time, adding that she couldn’t comprehend what had happened.
A statement from An Garda Síochána [Irish police] revealed on Wednesday reiterated their appeal for witnesses to what they believed was a ‘deliberate’ arson attack.
Chief Superintendent Anthony Petit of the Waterford/ Kilkenny Garda Division said: ‘There are people in the community who know the person or persons who carried out this arson attack in which four people have been murdered.
"If you have any information on the murderous arson attack, please speak to the Garda investigation team at Waterford Garda Station or contact the Garda Confidential Line on 1800 666 111 or speak with any member of An Garda Síochána."
* This article was originally published on Extra.ie.