The party said those who made the claims are “the same, faceless securocrats who murdered Pat Finucane”, referring to the Belfast solicitor whose death at the hands of loyalists in 1989 was found to have involved British state collusion.
“Irish Republicans do not attach any value to claims made in secret documents emanating from the British secret services, who were responsible for countless murders in Ireland during the course of the conflict,” a Sinn Féin statement said.
“This is not the first time that such unsubstantiated allegations have been made and Dessie Ellis rejects them as he has repeatedly done.”
The internal communication marked “secret” from the British embassy in Washington came after Mr Ellis was arrested in New York in 1982 in connection with immigration offences.
It states: “As you know, one of those arrested has turned out on investigation to be Desmond Ellis, who was arrested in Dublin in May 1981 for possession of electronic, remote-control devices.
“We understand that Ellis is linked by forensic evidence to some 50 murders in Northern Ireland and the Republic.”
Fine Gael has urged Mr Ellis to “come clean”.
The party’s chairman, Charlie Flanagan, said it was “utterly disgraceful” that an elected representative would respond to such allegations by saying they would not comment on anything said by the British.
“It shows a clear disregard, not only for the position Deputy Ellis holds, but also for the victims of our troubled past and the families they left behind,” said Mr Flanagan.
Sinn Féin can no longer deal with such matters in such a “routine way” he said. “We cannot simply forget the past and sweep the issues which strike at the very heart of the democratic process under the carpet to suit Sinn Féin.”
He pointed to the fact that the Sinn Féin leader, Gerry Adams, has called for an independent truth commission to resolve the conflicts of the past.
“If this is to be the case, let Dessie Ellis be the first person to be brought before it so that questions can be answered in respect of the 50 people murdered, to which the British government suggests he is in some way linked,” he said.