Stephen Harris, 34, had been in a critical condition in hospital after the accident that claimed the life of his brother Alan Harris, but died on Friday morning.
Both men were working on a sewer system  on Wednesday when it is believed they were overcome by toxic fumes.
Alan, the owner of the company carrying out the work, died at the scene.
Three members of Dublin Fire Brigade who attended the incident were treated in hospital and later discharged.
Two investigations, by police and the Health and Safety Authority, are continuing.
Robbie Keane, who plays his football in the United States with LA Galaxy, posted on Facebook on Thursday that "it's been a very difficult time for our family".
He is part of the Republic of Ireland squad that will take on Scotland in a Euro 2016 qualifier in Dublin on Saturday.
Another cousin of Republic of Ireland footballer Robbie Keane has died after a sewer accident in County Dublin.