It is feared Ciaran Cassidy, a 22-year-old shop worker with Irish parents, was on the same carriage as the Tube bomb that exploded under King's Cross Station.
Another Irish passport-holder, a woman in her 20s with dual Irish/New Zealand citizenship, is also among the missing.
Ciaran's father Seán, a native of Co Cavan, said the family are reconciling themselves to his likely death.
Seán Cassidy, a 57-year-old postman, said: "We are losing all hope now. I am sure he's dead, but the worst thing is waiting for confirmation."
There has been no sign of Ciaran among the injured in London's hospitals. His 53-year-old mother, Veronica, a Co Fermanagh native, said they had contacted all the hospitals and the emergency help-line, but Ciaran was not on any of the lists.
"All of his friends have been calling, desperate for news. I just want to find my son," she said.
His sister Lisa said they knew he had been on the Piccadilly Line, where one of the bombs exploded. She said her mother was "going frantic with worry. She can't bear the thought of him being dead down there in the tunnel. She will never recover from this."
Ciaran, who lived in Finsbury Park, north London, was on his way to his shop job in Holborn.
The alarm was raised when his boss rang his family to say he had not arrived.
Seán Cassidy, who moved to London 35 years ago, was due to return home for a holiday to Swanlinbar, Co Cavan, shortly but will concentrate instead on trying to locate his son.
The parents of the other missing Irish citizen are believed to have travelled from Auckland, New Zealand, to London in an attempt to locate their daughter, who moved there recently.
The Irish Department of Foreign Affairs would not identify the woman yesterday. New Zealand's Foreign Affairs spokesman James Kember said it would be some days before identification was possible.
She is believed to have been travelling on the Picadilly Line.
Another Irishman is said to be in hospital receiving treatment for injures, in addition to Tube passenger Willie John Walshe, a London-based 24-year-old architect from Moycullen, Co Galway, who was treated for burns and lacerations.
Mr Walshe said: "My hair was singed and I suffered flesh burns to my face. As terrible as it sounds, if the carriage wasn't so packed, I would have gotten it much worse."
Yesterday, Sky News highlighted a number of Irishmen among them Denis Brendan O'Shea from Cork and Adam Keogh and Anthony O'Brien as possibly missing, but the Irish Embassy has not been contacted by relatives.
Department of Foreign Affairs helpline: 1800 942548. Irish Embassy helpline: 02072012508/2501.