John Crotty, who manages the site for Cork County Council, revealed that the tunnels are under Bastion No 3, known as the old punishment block.
He said they are also aware of a 16th-century tunnel on the island which was used by smugglers, but have sealed the entrance to it at present because it would be unsafe to go in there until it is properly surveyed.
“We knew of the existence of the tunnels under Bastion No 3, but they are a lot bigger than we expected. They go on for hundreds of metres in a U shape. There’s a warren of areas within them for storage,” said Mr Crotty.
“We’re finding a lot of military equipment down there including parts for guns and badges for prisoners, which are more than 100 years old. There’s also a bellows, which formed part of a workshop and we are hoping to restore that and put it and other artifacts on display.”
He said these tunnels were probably built between the early and mid-1800s.
“The smugglers’ tunnel was used in the 16th century and runs from the back of the island to the middle of the island. We know where the opening to it is, but we have covered it up for health and safety reasons,” said Mr Crotty.
He said it is planned that UCC archaeologists would come to the island in the summer of 2018 to look “at excavation opportunities”.
Meanwhile, Mr Crotty and his staff are preparing for the arrival of a special guest tomorrow.
The Irish Prison Service is to hand over the Spike Island Convict Church chalice and paten.
They will go on display in the visitor centre.
The chalice and paten were created in 1848 just as Spike Island became a convict depot, spurred by the increasing prisoner numbers due to the famine years.
By the early 1850s the prison population had swollen to 2,300 inmates, making it the largest known prison in the world.
Religion played a large part in the reform of prisoners, and the chalice and paten would have been used in services involving tens of thousands of the most notorious murders and thieves in Britain and Ireland.
Tomorrow’s ceremony will also include the launch of the new book, A Souvenir Guide To The History Of Spike Island.
The visitor attraction has been shortlisted alongside the Guinness Storehouse in Dublin, Buckingham Palace, the Eiffel Tower, the Colosseum in Rome, and the Acropolis, for the European category in the Oscars of the travel industry.
A win for Spike Island would make it three in a row for Ireland.
Titanic Belfast won the title last year and the Guinness Storehouse the previous year.