Haulbowline (Irish: Inis Sionnach; Old Norse: Ál-boling) is an island in Cork Harbour off the coast of Ireland. The world's first yacht club was founded on Haulbowline in 1720. The western side of the island is the main naval base and headquarters for the Irish Naval Service, with the eastern side previously used for heavy industry and later redeveloped as a park.

Since 1966, the island has been connected to the mainland by a roadbridge.

The island's name may derive from Old Norse ál-boling or similar = "eel dwelling" ("area where there are conger eels"). The 17th and 18th-century spellings end in "-ing"; there may have been nautical influence on the spelling later. The Irish language name for the island, Inis Sionnach, translates to "island of the foxes".

At a strategic and deepwater position in the harbour, the island has long been a military base. The island was first fortified in 1602, and initially an important base for the British Army. In 1603 the Cork city fathers were accused of attempting to demolish it, and William Meade, the Recorder of Cork, was charged with treason as a result.

In 1720, much of the island was owned by the Cork Water Club, later connected to the Royal Cork Yacht Club (claimed as the world's first yacht club). There was a castle on the island which was the clubhouse for the Cork Water Club.

In 1806, the British Army moved to nearby Spike Island, and the fortifications were handed over, with 14 acres (6 ha) of land going to the Royal Navy, and the remaining 8 acres (3 ha) to the Board of Ordnance. (At the time the island was less than half its present size.)

An Ordnance Yard was established on the westernmost part of the island, separated from the rest by a large stone wall. A Martello tower was built for defence, and in 1808 a pair of magazines were erected on nearby Rocky Island with capacity for 25,000 barrels of gunpowder. East of the wall a Victualling Yard was laid out, and from 1807 to 1824 a series of buildings were constructed, several of which are still standing. Most prominent are six large Storehouses, three grouped together around a quay on the north side of the island, and three along what was then its eastern edge. Behind these was a large rectangular rainwater tank (which collected fresh water for the ships), on top of which was a quadrangular cooperage complex. To the south were a mast and boat store, at the top of a slipway, and to the west, along the length of the wall, were cottages and houses for the workers and officers of the yard. At this time, some 4 acres (2 ha) was added to the island through land reclamation, the first of several such additions.

Quite soon after so much investment, the Navy announced the closure of the Yard in 1831; ten years later, though, it reopened. The next major development came in the 1860s with the establishment of a Royal Navy Dockyard on Haulbowline, for warship repair and construction. To accommodate this new industrial complex the island was doubled in size to a total of around 60 acres (24 ha). The large basin was constructed, which today bisects the island, with a 408 ft (124 m) dry dock at one end (extended to 600 ft (180 m) just prior to the First World War). By the 20th century, the (now renamed) Royal Alexandra Victualling Yard area also contained a coaling/fuelling depot, as well as a naval hospital (housed in one of the Storehouses).