The drama began shortly after 4pm yesterday at Bachelor’s Walk near the Ha’penny Bridge, after the passer-by saw a man in the river.
He jumped in, but was soon in trouble himself. Both men were hauled to safety by rescue crews.
As the emergency services decided if the pair needed hospital treatment, the first man jumped in again.
Gardaí in a dinghy, who had arrived on the scene, had to fish out the man a second time. A garda spokesperson said one man was detained under the Mental Treatment Act.
“There’s a lot of confusion as to what exactly happened, but we are just glad that no one drowned. We are getting the poor fellow looked after,” the spokesperson said.
The man who jumped in was said to be in an incoherent state when taken into custody and was expected to be seen by a doctor last night.
Dozens of people jump or fall in to the River Liffey every year, many of them in the area between O'Connell Bridge and the Ha’penny Bridge.