There are 27 mammal species native to Ireland or naturalised in both the Republic of Ireland and Northern Ireland before 1500. The Red List of Irish terrestrial mammals was updated in 2019, with assessments of these 27 species. One species is locally extinct, one is vulnerable and 25 are least concern species. Not assessed were nine mammal species that were introduced after 1500. The following tags are used to highlight each species' conservation status as assessed on the IUCN Red List: NE indicates not endemic

Order: Diprotodontia (kangaroos, wallabies, wombats and allies)

Though most marsupials make up a great part of the fauna in the Australian region, the red-necked wallaby has been introduced to Ireland: a population is currently breeding on Lambay island.

Order: Rodentia (rodents)

Rodents make up the largest order of mammals, with over 40% of mammalian species. They have two incisors in the upper and lower jaw which grow continually and must be kept short by gnawing.

Order: Lagomorpha (lagomorphs)

The lagomorphs comprise two families, Leporidae (hares and rabbits), and Ochotonidae (pikas). Though they can resemble rodents, and were classified as a superfamily in that order until the early 20th century, they have since been considered a separate order. They differ from rodents in a number of physical characteristics, such as having four incisors in the upper jaw rather than two.

Order: Erinaceomorpha (hedgehogs and gymnures)

The order Erinaceomorpha contains a single family, Erinaceidae, which comprise the hedgehogs and gymnures. The hedgehogs are easily recognised by their spines while gymnures look more like large rats.

Order: Soricomorpha (shrews, moles, and solenodons)

The "shrew-forms" are insectivorous mammals. The shrews and solenodons closely resemble mice while the moles are stout-bodied burrowers.

Order: Chiroptera (bats)

The bats' most distinguishing feature is that their forelimbs are developed as wings, making them the only mammals capable of flight. Bat species account for about 20% of all mammals.

Order: Cetacea (whales)

The order Cetacea includes whales, dolphins, and porpoises. They are the mammals most fully adapted to aquatic life with a spindle-shaped nearly hairless body, protected by a thick layer of blubber, and forelimbs and tail modified to provide propulsion underwater.

Order: Carnivora (carnivorans)

Carnivorans include over 260 species, the majority of which eat meat as their primary dietary item. They have a characteristic skull shape and dentition.

Order: Artiodactyla (even-toed ungulates)

The even-toed ungulates are ungulates whose weight is borne about equally by the third and fourth toes, rather than mostly or entirely by the third as in perissodactyls. There are about 220 artiodactyl species, including many that are of great economic importance to humans.

Locally extinct

The following species are locally extinct in the country:

Comparison with Great Britain

The following species are found in Great Britain but not in Ireland: