The flooded valley is 16 kilometres (9.9 mi) long, and in the centre over 42–45 metres (138–148 ft) deep; the sea level is higher outside its mouth, as is normal for a fjord.

The river itself does not enter on the main line of the Harbour but curves northwards and then runs south into the inlet.

On that northern shore lies the mountain Mweelrea, Connacht's highest, rising to 814 metres (2,670 ft), and the sides of which fall steeply into the inlet, with Uggool Beach at its western edge.

[8] A key point in such argument is that it lacks extensive steep cliff walls, but this does not take full account of the underwater sides of the valley, and the shape of its bed.

Irish naturalist, writer, and historian Emily Lawless, in an essay from 1898, recalled renting a cottage by the 'Greater Killary' on a research trip, in order to "extract .

Leenaun has a hotel and guesthouses and there are one more of each at Delphi, in a valley running down to Killary Harbour from the north.

This building was formerly a modest house which was used by Ludwig Wittgenstein, the famous philosopher, as a quiet place to write shortly after World War II.

Near to Rosroe begins the so-called Green Road, a rough roadway running along the side of the valley back east towards Leenaun.

[citation needed] Aquaculture is also important locally with a salmon farm based at Rosroe while mussel rafts are a common sight further to the east.