Sean McGrotty, 46, died along with his two sons Mark, 12, and Evan, eight, their grandmother Ruth Daniels, 57, and her 14-year-old daughter Jodie Lee Daniels, when their SUV sank after sliding off a slipway in Buncrana, Co Donegal.
It is understood the vehicle lost grip on a thick blanket of algae coating the pier's slipway and slid into the water; the victims were on a day out from nearby Derry.
The only survivor of Sunday night's accident was Mr McGrotty's four-month-old baby girl Rionaghac-Ann
Davitt Walsh has been hailed a hero after swimming out into the harbour in an effort to save the six occupants of the Audi Q7.
Mr McGrotty handed him his baby out of the broken driver's side window just moments before the vehicle sank.
Her mother Louise has said the baby is her only reason to go on living, having lost her partner, two sons, her mother and sister.
The funeral will be held at Holy Family Church, near the family home in the Ballymagroarty area of Londonderry.
Fr Paddy O'Kane, who's leading today's service, said Louise went to see him after the tragedy: "I just held her in my arms and said I'm so sorry, and words wouldn't come to me.
"I kissed her on the cheek, I tasted the salt of her tears - she had been obviously crying so much.
"And I just sat with her, and stayed with her."
