The City of Cork is expecting to welcome 100,000 visitors a year to Spike Island by 2020, following €6m (US$6.7m, £5.2m) of renovations to turn the fort and former jail into a tourist attraction.

The 103 acre (416,000sq m) island in Cork Harbour, dominated by the 18th-century Fort Mitchel, underwent tourist-friendly renovations after Cork Council took over management from the Department of Defence in 2010.

Spike Island is most famously known as a prison, with thousands of its inmates transported from there to Australia in the 19th century. The site was originally a monastic settlement in the 7th century, with its first artillery fortification built in the 18th century.

“A lot of people don’t realise that Spike Island was the largest prison in the world in 1850,” said John Crotty, general manager of Spike Island, speaking to Irish Times. “It was the largest prison in Britain and Ireland before and ever since. There has never been anything bigger; it was much bigger than Alcatraz, for example, so it has a massive story to tell.”

Among the works carried out on the island, a new pontoon has been installed for better tourist access, while the Goal Punishment Block and Shell store have been refurbished for public exhibition. Additionally the a new Military Exhibition Yard has been built and the island’s café has been extended.

The refurbishment is being carried out in multiple phases, with future plans add an aquarium and a museum within the existing fort structures, while plans to install self-catering accommodation are also on the cards.