Inishowen has several harbours, some of which are used for commercial fishing purposes, including Greencastle, Bunagee and Leenan. A seasonal ferry service crosses the Foyle, connecting Greencastle with Magilligan in Derry, while another crosses the Swilly, connecting Buncrana with Rathmullan. The village of Fahan has a privately built Marina.
There are several small outlying islands off the Inishowen coast, most notably Inishtrahull and Glashedy islands, both uninhabited, although the former was inhabited until the early twentieth century. Inch, located in Lough Swilly is technically no longer an island, as it has a causeway connecting it to the mainland at Tooban, south of Fahan.
Lough Swilly is a fjord-like lough, and was of strategic importance for many years to the British Empire as a deep-water harbour. It is also famous as the departure point of the Flight of the Earls. Lough Foyle is important as the entrance to the river Foyle, and the city of Derry, but is much more shallow than Lough Swilly, and requires the use of a guide boat to guide ships to and from the port of Derry.
A large area of land, most of which now forms part of Grianán Farm, one of the largest farms in Ireland, was reclaimed from a shallow area of Lough Swilly, stretching from the village of Burnfoot to Bridgend and Burt. The outline of this land is plainly visible due to its flatness proving a marked contrast to the more mountainous area surrounding it.
Predating the formation of Donegal by centuries, the area was named Inis Eoghain (the Island of Eoghan) after Eoghan, son of Niall of the Nine Hostages (Niall Naoigeallach, a High King of Ireland), whose name was also used for Tyrone (Irish: Tír Eoghain). Inis Eoghain is also the ancient homeland of the Mac Lochlainn clan (descended from the tribe of Eoghan), a clan that grew so formidable that they eventually came under siege by a Limerick King, who came north to Aileach, and ordered the destruction of Aileach fort, and that each soldier was to carry away a stone from the fort in order to prevent its rebuilding. Later, after the decline of the Mac Lochlainn clan, the chieftainship of Inis Eoghain was usurped by the Ó Dochartaigh clan, as they lost their own homeland in the Laggan valley area of Tír Conaill.
Inishowen has many historical monuments, dating back to early settlements, and including the ruins of several castles, and the fort at Grianán Aileach. The ancient Grianán Ailigh fort at Burt was the one time seat of the High Kings of Ireland, including both High Kings of the Mac Lochlainn Clan, who held power in Inis Eoghain for many centuries. It was restored in the nineteenth century, although some damage in recent years has resulted in the partial collapse of the south side wall. Among the main castle ruins of Inishowen are Carrickabraghey on the Isle of Doagh, the Norman Castle at Greencastle, Inch Castle, Buncrana Castle and Elagh Castle.
In 1196, John de Courcy, a Norman knight who had invaded Ulster in 1177, defeated the King of the Cenél Conaill and most of Donegal was at his mercy. Two years later he returned to devastate Inishowen.