Five members of a family died on Sunday when their car left Buncrana pier in County Donegal, entering Lough Swilly.
A baby girl was saved by a passer-by moments before the car sank.
The funerals of those who died will take place in Londonderry on Thursday.
The victims were Ruth Daniels, 57, her daughter Jodie Lee Daniels, 14, Sean McGrotty, 49, and his sons Mark, 12, and Evan, eight.
They were the mother, sister, husband and two sons of Louise McGrotty, from Londonderry.
Only four-month-old Rionaghac-Ann survived.
The victims' family has praised Davitt Walsh, the County Donegal man who swam to the car and rescued her.
Flags on public buildings in the Republic of Ireland are also at half-mast for those killed in blasts in the Belgian capital Brussels on Tuesday.
More than 30 people died and 250 were injured in explosions at the city's airport and a metro station, with the so-called Islamic State group claiming it was behind the attacks.
The Irish national flag is flying at half-mast over Dublin's Government Buildings as a mark of respect to victims of a drowning tragedy on the country's north-west coast.