Dublin Bay stretches 10 kilometers from Howth Head south to Dalkey Island, enclosing 200 square kilometers of protected water that has sheltered Viking longships, Georgian packet steamers, and twentieth-century fishing trawlers. The bay's northern anchor, Howth peninsula, rises 171 meters above sea level in sheer quartzite cliffs that Irish and Norse navigators used as a landmark for a millennium. Ireland's Eye, the uninhabited island one kilometer offshore, was named Eria's Island by Norse settlers in the ninth century — the corrupted Gaelic "Inis Ereann" later became the anglicized Eye. Its Martello tower, built in 1804 during Napoleonic invasion fears, stands on the same rock where seventh-century monks maintained a hermitage and carved stone crosses that remain visible today. The bay's ecological importance earned it Special Protection Area status in 1988 under European Union directives. Grey seals from the Lambay Island colony patrol the northern waters year-round, while autumn and winter bring Brent geese from Arctic Canada to feed on zostera beds along the shoreline. Howth's eastern cliffs host Ireland's largest mainland gannet colony, established in 2008 when breeding pairs migrated from overcrowded Lambay. The limestone sea stacks at the peninsula's nose, carved by 12,000 years of Atlantic swells, now support kittiwakes, razorbills, and guillemots from March through August. Local naturalists documented 89 seabird species in bay waters between 2020 and 2025, making the protected zone one of Dublin's richest ornithological sites. Dublin Bay's maritime heritage runs deeper than tourism. Howth Harbor, completed in 1807 after a four-year construction effort, was designed to replace the silted Ringsend port and serve as Dublin's primary passenger gateway. The harbor's 1,200-meter east pier and 850-meter west pier enclosed 16 hectares of water, but prevailing easterlies caused persistent siltation that forced the mail packet service to relocate to Dún Laoghaire in 1833. The working fishing fleet that replaced passenger ships still operates from the harbor's inner basin, landing mackerel, Dublin Bay prawns, and crab. The harbor's East Pier lighthouse, automated in 1982, continues to guide vessels with a white flash every five seconds visible 18 kilometers out to sea. Coastal navigation around Ireland's Eye requires attention to the Rowan Rocks submerged reef on the island's northeast side, marked by a north cardinal buoy since 1997.