Great Island (Irish: An tOileán Mór) is an island in Cork Harbour, at the mouth of the River Lee and close to the city of Cork, Ireland. The largest town on the island is Cobh (called Queenstown from 1849 to 1920). The island's economic and social history has historically been linked to the naval, ship-building, and shipping activities in the town's environs.

In the early 21st century, a different marine industry has arisen. Tourism and related services have become a significant part of the local economy, including cruise ships attracted by the Port of Cork berthing facilities on the island. This is the only dedicated cruising ship berth in the Republic of Ireland.

The island has an area of 53.1 km² (13,100 acres), making it Ireland's second-largest after Achill.

The ancient name of the island was Ard-Neimheadh, the "High (or important) island of Neimheadh". According to the 11th-century work Lebor Gabála Érenn, Neimheadh was the leader of a group who invaded the area in prehistoric times. Archaeological evidence of prehistoric and early medieval settlement on Great Island is recorded on the Record of Monuments and Places of the National Monuments Service. This includes records of ringfort, fulacht fiadh, holy well, and bee bole structures.

By the early 13th century, the island was under the control of the Anglo-Norman Hodnett family. The Hodnetts built several structures, including Belvelly Castle, to defend the island and its approaches. The island and its defenses were taken by the de Barra (Barry) family in the 14th century. The Barry family (later Earls of Barrymore) retained power in the area for several centuries, and the island became known as "Barrymore Island" or Oileán Mór an Bharraigh, meaning "Great island of the Barrys".

In the 18th and 19th centuries, due to the strategic importance of the island within Cork Harbour (overlooking the naval berths and military bases at Haulbowline and Spike Island), several coastal defence fortifications were built on Great Island. These included construction by the British of Cove Fort (between Cobh and Cuskinny) in the 18th century and several Martello towers (at Belvelly, Monning and Rossleague) in the 19th century.

The largest settlement on Great Island, Cobh, had significant development during the 19th and early 20th centuries, stimulated by additional expansion in shipping and naval activity in the area.

Great Island is located in Cork Harbour on Ireland's south coast, and is made-up of the civil parishes of Clonmel and Templerobin.