Jumping into the River Liffey has been a dangerous pastime for Dubliners for centuries. Some do it for kicks, others for bets and others just to cool down during hot summer days. A quick scan of The Irish Times archive showcases the long running (and often deadly) activity.
An Irish Times article from March 1890 relates the story of a Miss Marie Finny, “a professional swimmer” who was arrested just before she attempted to jump into the river off O’Connell Bridge. 
In 1909, a hotel porter called Hugh Bernard McGrath was rescued from the Liffey after he got into difficulty swimming after jumping from the eastern parapet of O’Connell Bridge. 
A “strange affair” was reported in 1932 which concerned an “unknown man” who was seen swimming in the liffey late one Monday evening. It was reported did not “take any notice” of two life buoys that were thrown towards him or a boat that passed. He soon got into difficulty and drowned. 
In 1939, a soldier named James Donlan (25) “disappeared” while swimming in the Liffey. It took a extensive search operation to find his body. 
The body of Michael Kinsella, 35, a labourer in the Guinness brewery was found in the Liffey in 1954. It was believed that he entered the river “to settle a wager”. 
There were also cases of young men drowning in the Liffey in August 1968, January 1977 and December 1986.
In 1994, a Scottish tourist drowned after trying to swim across the Liffey in the early hours of Saturday morning. 
As you can see from the youtube clips below, jumping into the Liffey is as popular as ever. (Come Here To Me! does not reccomend it.)
1. Anon, Attempt to jump from O’Connell Bridge into the Liffey, The Irish Times Saturday, March 29, 1890
2. Anon, Rescue from the Liffey, The Irish Times, Saturday, July 10, 1909
3. Anon, Man Drowned In The Liffey, The Irish Times, Tuesday, January 19, 1932
4. Anon, Soldier Drowned In The Liffey, The Irish Times, Friday, August 18, 1939
5. Anon, Swimmers Body Taken From Liffey, The Irish Times, Monday, November 15, 1954
6. Anon, Tourist Dies Trying To Swim Liffey, The Irish Times, Monday, May 30, 1994