Dublin Bay (Irish: Cuan Bhaile Átha Cliath) is a C-shaped inlet of the Irish Sea on the east coast of Ireland. The bay is about 10 kilometres wide along its north-south base, and 7 km in length to its apex at the centre of the city of Dublin; stretching from Howth Head in the north to Dalkey Point in the south. North Bull Island is situated in the northwest part of the bay, where one of two major inshore sand banks lay, and features a 5 km long sandy beach, Dollymount Strand, fronting an internationally recognised wildfowl reserve. Many of the rivers of Dublin reach the Irish Sea at Dublin Bay: the River Liffey, with the River Dodder flow received less than 1 km inland, River Tolka, and various smaller rivers and streams.
The metropolitan area of the city of Dublin surrounds three sides of the bay (the north, west, and south), while the Irish Sea lies to the east. Dublin was founded by the Vikings at the point where they were able to ford the River Liffey with the first wattle bridge up from the estuary. The city spread from its birthplace, around what is now the James's Gate area, out along the coastline, northeast towards Howth and southeast towards Dalkey.
The bay is rather shallow with many sandbanks and rocky outcrops, and was notorious in the past for shipwrecks, especially when the wind was from the east. Until modern times, many ships and their passengers were lost along the treacherous coastline from Howth to Dun Laoghaire, less than a kilometre from shore. Early maps of the bay carefully show narrow shipping channels and mooring areas.
The bay had two inshore sand banks, the North Bull and the South Bull. With the building of the Bull Wall, the North Bull began to build up rapidly, forming North Bull Island (often simply "Bull Island"). A southern wall had earlier been built - the Great South Wall - but did not result in island formation, the South Bull remaining today an area of mud flats and strand. In addition there are several offshore sandbanks, notably Kish Bank (on which a lighthouse stands).
From north to south, Dublin Bay features beaches at Sutton Strand, Dollymount Strand on North Bull Island, Sandymount, Seapoint and south of Dun Laoghaire. The remaining coast is either rocky (with cliffs on Howth Head, for example) or mud coming up to sea walls. In most parts, the land slopes gently down to the sea, but aside from Howth Head, there are bluffs along much of the Raheny coastline, and the sharper slopes just inland at Monkstown and Old Dunleary.
Over 500 crew and passengers (mostly military personnel) were lost when the steamship RMS Leinster was torpedoed and sunk by German U-Boat UB-123 on 10 October 1918. She lies in 33 metres (108 ft) of water at .
In 1972, the Dublin Port and Docks Board proposed building an oil refinery in Dublin Bay. The plan was vigorously opposed by environmentalists, including Dublin City Councillor Seán D. Loftus, on the grounds that it posed a serious risk of pollution. Loftus, a lifelong campaigner for Dublin Bay, changed his name by deed poll to "Seán Dublin Bay Loftus" when standing for election to the Dáil. Although he was not elected, he succeeded in publicising the issue and the proposal was eventually turned down by the Minister for Local Government, James Tully. (Loftus later changed his name by deed poll to "Seán Dublin Bay Rockall Loftus" as part of a campaign to press the Irish Government to make a territorial claim to the Rockall islet off the coast of County Donegal). Loftus also led opposition to the 2002 and subsequent applications by the Dublin Port Company to fill in 52 acres (210,000 m2) of Dublin Bay. Other suggestions for the bay have included a proposal to build giant underwater gas storage tanks, and to infill the near-lagoon behind North Bull Island to form a leisure park.
In the summer of 2010, An Bord Pleanála refused permission to the Dublin Port Company to proceed with its plans to infill a further 52 acres (210,000 m2) of Dublin Bay. The proposed infill, which has been vehemently opposed by residents, politicians, environmentalists and others around the bay for over 20 years, was refused on one point. An Bord Pleanála rejected nine out of ten of its own inspector's recommendations for refusal, but refused permission on the basis that it was not satisfied that the proposed development would not adversely affect the integrity of the South Dublin Bay and River Tolka Estuary proposed Special Protection Area and adversely affect the natural heritage of Dublin Bay. Within a few months of the decision, the Dublin Port Company applied for and received a pre-application meeting with An Bord Pleanála. The Dublin Port Company has redrafted their proposal in relation to the SPA boundary and may resubmit an application for the project.
Coastal flooding can occur at high tide at several points, notably the city side of Clontarf and Sandymount in strong easterly winds.
James Joyce set much of the action in his novel Ulysses around the bay, from the Forty Foot bathing place—in which the character Buck Mulligan washed on Bloomsday morning—to Howth, where Leopold Bloom made love to his wife Molly under the rhododendrons.
Dublin Bay Sailing Club was founded in 1884 under the title 'The Sailing Boat Club' to provide racing for amateurs racing small open boats in Scotsman's Bay on the south side of Dublin bay between Kingstown Harbour and Sandycove Harbour. The club expanded quickly, and this caused a change of starting location to the mouth of Kingstown Harbour in 1889. The racing of course took place throughout the bay. In 1925, the starting line was brought into Dun Laoghaire Harbour, to the Carlisle Pier. Today there are about 1,100 members, and racing is generally limited to the southern half of the bay.
Other yacht clubs in the bay include:
Sutton Dinghy Club (Sutton Creek on the North side).
Clontarf Yacht and Boat Club (Clontarf on the North side).
Poolbeg yacht Club (Poolbeg in the River Liffey).
Water Wag Club (Dun Laoghaire). 
Irish National Sailing Club. (Dun Laoghaire)
Dun Laoghaire Motor Yacht Club.
Royal Irish Yacht Club (Dun Laoghaire).
Royal St.George yacht Club (Dun Laoghaire).
National Yacht Club (Dun Laoghaire).
- "Dublin Bay infill plan rejected". RTÉ. 2010-06-09. Retrieved 2011-04-20.
- "Bay Watch says 'No' to plans for a 52-acre infill". Herald.ie. 2008-10-29. Retrieved 2011-04-20.
- "Dublin Bay - Proposed 52 acre infill". Finianmcgrath.ie. 2008-06-15. Retrieved 2011-04-20.
- "Dublin Port Must Abandon Plan to Infill 52 Acres of Dublin Bay - Bruton". Richardbruton.ie. 2009-11-18. Retrieved 2011-04-20.
- "Dublin Port expansion plan refused". The Irish Times. 2010-06-09. Retrieved 2011-04-20.
- "BirdWatch Ireland welcomes rejection of proposal to infill part of Dublin Bay". BirdWatch Ireland - South Dublin Branch. 2010-06-09. Retrieved 2011-04-20.
- "Say NO to the 52 Acre Infill of Dublin Bay". Clontarf Residents' Association. 2010. Retrieved 2011-04-20.
- Dublin Bay a century of sailing 1884-1884 by Donal O'Sullivan
- The Water Wags 1887-2012 by Alf and Vincent Delany
- Dublin Bay - Proposed 52 acre infill
- Satellite photo of County Dublin (NASA)
- Dublin Bay dive sites @ Trinity College Dublin
- List of shipwrecks: Dublin Bay to Gormonstown