The archipelago stretches for roughly 7 kilometres (4.3 mi) from the island of Bac Beag in the south towards Cairn na Burgh Beag to the north east. The largest island in the group is Lunga, which is 7 kilometres (4.3 mi) west of Gometra, 6 kilometres (3.7 mi) southwest of Rubha' a' Chaoil on Mull, 12 kilometres (7.5 mi) south east of Coll, and 5 kilometres (3.1 mi) north west of Staffa.
Other larger islands in the group are Cairn na Burgh Mòr, Fladda and Bac Mòr. There are numerous smaller skerries, particularly north of Lunga. All the islands are of volcanic origin.
There are several possible duns on the islands of Iron Age origin. Little is known of the early history, although these prominent landmarks would have been significant waypoints for the Norse settlers during their conquest in the early years of the Kingdom of the Isles, which lasted from the 9th to the 13th centuries.
Treisinis is a Gaelic name of unknown meaning that includes the Old Norse root ness, meaning "headland".
Bac Mòr is also known as "The Dutchman's Cap" in English due to its shape. Bac Beag is from the Gaelic Am Baca Beag, with Bac meaning "bank" or "dune" and Beag simply being a diminutive in contrast to Mòr. Bac Mòr is also known as Baca Rois in Gaelic, which means "dune of the Ross" and is a reference to the shape of the island when framed against the Ross of Mull. Lunga is probably from the Old Norse lungr-øy, meaning "longship island". Sgeir an Eirionnaich and Sgeir a' Chaisteil are Gaelic names meaning "Irishman's skerry" and "castle skerry" respectively. Cairn na Burgh Mòr means "cairn of the large fort" and Cairn na Burgh Beg is thus "cairn of the small fort".
The name Fladda originates from the Old Norse for "flat island".
The Treshnish Isles from above Port Haunn on Mull. Nearest are Cairn na Burgh Beag and Cairn na Burgh Mòr. Behind are the low flat island of Fladda and the tallest island, Lunga, which obscures Bac Mòr and Bac Beag beyond.
The Treshnish Isles from the east as seen from Loch Tuath. From left to right: (Ellean Dioghlum), Bac Mòr, Lunga, Fladda, Cairn na Burgh Mòr, Cairn na Burgh Beag.