Two Irish tourists have been fined after stripping down to their undies and jumping from a ferry in Sydney Harbour, police say.

The men, aged 20 and 24, swam to the Sydney Opera House and made it onto dry land shortly after 8pm last night.

The pair, who had spent the evening at Manly, dashed towards Circular Quay but were met by a contingent of police and arrested.

They were fined $200 ($249) each for leaving the ferry in an unauthorised manner.

Acting Inspector Craig Hansen said the men were a little worse for wear.

"A little bit cool and worse for wear, (they) were detected by police in their underwear just near the Sydney Opera House," he told the Seven Network.