Enda Kenny has said he will never forget the chilling eyewitness accounts of the Buncrana pier tragedy.
As the remaining family of the victims await the release of their bodies to finalise funeral arrangements, the Dail held a minute's silence in their memory.
Acting Taoiseach Mr Kenny said the scale of the horror put everything else in perspective.
"Our hearts go out to them and what is a devastating impact on the lives of the extended family," he said.
"We all know in our own lives the numbing grief that comes with the loss of a loved one.
"In these circumstances, however, this is a particular and deepest grief, that the tranquillity and beauty of a sunny spring Sunday was shattered by this tragedy."
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.
The only survivor was Mr McGrotty's four-month-old baby girl Rionaghac-Ann.
Hero rescuer Davitt Walsh swam out into Buncrana harbour in north Co Donegal on Sunday night in an effort to reach the six people trapped in a car that had slid off the slipway into Lough Swilly.
Mr McGrotty handed him his baby out the broken driver's side window just moments before the Audi Q7 sank.
Mr Kenny said: "I will recall - as many people will - forever the words of (eye-witness) Francis Crawford in his description of what happened.
"And I admire the courage of Davitt Walsh who rescued and saved life of baby Rionaghac-Ann.
"When you try to consider the horrendous impact of what was happening as that vehicle skid towards the water.
"A father handing his own baby to the rescuer - to say 'save our baby' - this puts things into perspective and in context."
Four-month-old Rionaghac-Ann is said to be in a stable condition in Letterkenny University Hospital.
Fianna Fail leader Michael Martin described it as an unspeakable tragedy and trauma.
"The nation is truly shocked at the scale of that tragedy and its wider impact on the communities of Donegal and Derry," he added.
Sinn Fein leader Gerry Adams said the thoughts and prayers of everyone are with the family.