It is located very far north from the rest of Essos but is far too small to be considered a continent in its own right, so it is often loosely considered part of Essos. It is at roughly the same longitude as Vaes Dothrak and Qarth. Despite these vast distances, the whale-hunting, Inuit-like Ibbenese travel very far across the world oceans, trading across much of Essos and sometimes in Westeros as well. The Port of Ibben is on its south-eastern coast.
In the books
In the A Song of Ice and Fire novels, Ibben (also sometimes called Ib) is home to a hardy race of sailors. The Ibbenese are an Inuit-like people adapted to the cold, frequently described as hairy and short. They chew whale blubber to keep their metabolism high in the cold temperatures. They principally trade in whale blubber and meat.
The island's principal city is the Port of Ibben, located on its south coast. The northern part of the island is cold and sparsely-inhabited. Mammoths are said to exist on the island. The Ibbenese have also colonized several other nearby islands, including Far Ib, and even have some settlements on the remote northern coast of Essos.
Author George R.R. Martin has stated that the island of Ibben is roughly the same size as real-life Iceland.
The Lands of Ice and Fire (a companion map collection for the novels, not the TV series, released in 2012) seems to indicate that the island as a whole is officially named "Ib", and the major port city on it is named "Ibben"; in which case, the island is just often referred to as "Ibben" by extension.