Prior to the 20th century, the island was more commonly called Inis Bant or as Árainn na Naomh. The modern Irish name, Árainn Mhór, (which translates as "Great Aran" in English) leads to some confusion with Arranmore, County Donegal. The Irish word Árainn means "long ridge", presumably referring to the island's geography. Árainn is the legal placename in Irish or English as declared in the Official Languages Act 2003.
The island is an extension of the Burren. The terrain of the island is composed of limestone pavements with crisscrossing cracks known as "grikes", leaving isolated rocks called "clints". The limestones date from the Visean period (Lower Carboniferous), formed as sediments in a tropical sea approximately 350 million years ago, and compressed into horizontal strata with fossil corals, crinoids, sea urchins and ammonites. Glaciation following the Namurian phase facilitated greater denudation. The result is that Inis Mór and the other islands are among the finest examples of Glacio-Karst landscape in the world. The effects of the last glacial period (the Midlandian) are most in evidence, with the island overrun by ice during this glaciation. The impact of earlier Karstification (solutional erosion) has been eliminated by the last glacial period. So any Karstification now seen dates from approximately 10,000 years ago and the island Karst is thus recent.
Solutional processes have widened and deepened the grikes of the limestone pavement. Pre-existing lines of weakness in the rock (vertical joints) contribute to the formation of extensive fissures separated by clints (flat pavement like slabs). The rock karstification facilitates the formation of sub-terrainean drainage.
The island has an unusually temperate climate. Average air temperatures range from 15 °C in July to 6 °C in January. The soil temperature does not usually drop below 6 °C, although the end of 2010 recorded a prolonged period of snow, the first in living memory. Since grass will grow once the temperature rises above 6 °C, this means that the island (like the neighbouring Burren) has one of the longest growing seasons in Ireland or Britain, and supports a diverse and rich plant life. Late May is the sunniest time, and also likely the best time to view flowers, with the gentians and avens peaking but orchid species blooming late.
The island supports arctic, Mediterranean and alpine plants side by side, due to the unusual environment. Like the Burren, the Aran islands are known for their unusual assemblage of plants and animals. The grikes (crevices) provide moist shelter, thus supporting a wide range of plants including dwarf shrubs. Where the surface of the pavement is shattered into gravel, many of the hardier Arctic or alpine plants can be found. But when the limestone pavement is covered by a thin layer of soil, patches of grass are seen, interspersed with plants like the gentian and orchids. Insects present include the butterfly the pearl-bordered fritillary (Boloria euphrosyne), brown hairstreak (Thecla betulae), marsh fritillary (Euphydryas aurinia) and wood white (Leptidea sinapis); the moths, the burren green (Calamia tridens), Irish annulet (Gnophos dumetata) and transparent burnet (Zygaena purpuralis); and the hoverfly Doros profuges.
Inis Mór today is a major tourist destination, with bed and breakfast accommodation scattered across the island. Private minibuses, horse-drawn carriages and bicycles are the main methods of getting about for the numerous tourists who visit the island in the summer months.
There is a small museum illustrating the history of Dún Aonghasa and its possible functions, while the Aran Sweater Market is also a focal point for visitors who can trace the culture and history associated with the Aran sweater through the on-site museum. Nearby are a Neolithic tomb and a small heritage park at Dún Eochla, featuring examples of a traditional thatched cottage and poteen distillery. The Tempull Breccain (Church of Brecan), commonly called the Seven Churches of Aran, is a complex of churches and other buildings dedicated to the 5th-century Saint Brecan, once a popular destination for pilgrims. There is a nineteenth-century lighthouse in the centre of the island, said to have views of the mountains of distant County Kerry on a clear day.
The island plays host to Ted Fest each year. Established in 2004 it is a celebration of the television sitcom "Father Ted." Festival goers dress as their favourite characters, watch their favourite episodes and take part in various Ted related events and competitions.
Some of the limestone sea cliffs have attracted interest from rock-climbers. Diving is possible. A particularly popular location for this is Poll na bPéist (hole of worms/sea monsters), located at the southern coast of the island, which is a large naturally formed rectangular pool communicating via underground channels with the sea. Since 2012 Inis Mór has hosted an event as part of the Red Bull Cliff Diving World Series.
The island is serviced by Aran Ferries ferry from Rossaveal and Doolin and by Aer Arann Island from Inishmore Aerodrome to Connemara Airport.
The island features heavily in Martin McDonagh's play The Lieutenant of Inishmore.
It was also featured in the 1997 film The Matchmaker. The first story in These Precious Hours by Michel Corrigan has a scene set on Inis Mór.
Annals of Inisfallen (AI)
The table below reports data on Inishmore's population taken from Discover the Islands of Ireland (Alex Ritsema, Collins Press, 1999) and the Census of Ireland.
AI=Annals of Inisfallen. (AF)M=Annals of the Four Masters.