The Taoiseach, the Minister for Transport and the Lord Mayor of Dublin were among the first to sign a book of condolence opened this morning for the victims of the Dublin Bus crash at the weekend.
Dublin Bus opened the book at its head office to allow members of the public express their sympathy for the five people who died and for the injured, some of whom are still being treated in hospital.
The company also took advertisements in the press expressing sympathy on behalf of its staff and management to those involved in the tragedy.
Mr Brennan was attending the emergency meeting of the board of Dublin Bus, which is continuing.
A slow trickle of people arrived at the O'Connell Street offices as word emerged about the book of condolence. The small ticket office on the ground floor was also packed with reporters and there were a number of television cameras.
Ms Maire Dodd from Bray said she had been in Athy on Saturday when she heard about the accident. She often walks up the quays and said she was shocked at the fact that people going about their daily business had been killed.
"It's an awful thing. I feel very sorry for everyone involved in it."