They have spoken to several teenage suspects over the last 48 hours and have identified two areas where the fire was set, gutting the 18th century property on Sunday.
Superintendent Charlie Barry, who is overseeing the probe, said gardaí are satisfied a gang of between 20 and 40 teenagers were within the surrounds of the historic house and its grounds around the time of the fire.
He said they have identified a number of individuals, ranging in age from 15 to 17, and have called to several addresses on the southside of the city to speak to youngsters over the last 48 hours.
Gardaí have taken several statements and are continuing their investigations.
Supt Barry said: “We are confident that we are making significant progress in our investigation. I would urge anyone with information to come forward at this stage and talk to us. It is in their interests to come forward, sooner rather than later.”
The news came as the Grange Frankfield Partnership, which has been campaigning for almost seven years to save the property, spoke out yesterday to oppose demolition of the fire-ravaged structure.
Spokesman Ger Lehane said they believe the house is an “active protected structure, capable of consolidation and retention”.
“We strongly oppose any consideration of demolition pending a full assessment of the possibilities of stabilising the structure,” he said.
“We consider that we owe it to the house, to history, to our community, and to the efforts of ourselves and our supporters since we commenced our campaign over six years ago, to resist any attempts to reduce the site to Cork’s Ground Zero, which would only serve as an indictment of our society, and a monument to its abject failure to protect our priceless heritage.
“We cannot contemplate the disappearance of an iconic landmark that has been a feature of the landscape of the southside of Cork City for over 230 years, and which would be an important element in the landscape of the Tramore Valley Park, to have its memory consigned to oblivion.”
Mr Lehane said there are many examples of successful consolidation and restoration of fire-damaged structures of major architectural, cultural, and historical importance.
The pertnership plans to make a submission to Cork County Council opposing demolition. The Irish Georgian Society is expected to make a similar submission.
Mr Lehane said they have been greatly encouraged by the widespread political condemnation of the destruction caused by the fire, and by the groundswell of support from various sources.
Meanwhile, county councillor Marcia D’Alton (Ind) has called on the local authority to pursue the owners of Vernon Mount for prosecution under section 58 of the Local Government (Planning & Development) Act 2000.
In her motion tabled for the next full council meeting in early September, she has asked officials to identify the details of notices served under section 59 of the Planning and Development Act 2000 on the owners since 1997, the dates when they were served, and details of any works undertaken in response to those notices.