[5][6][7][8] The first scientific descriptions of the animal's remains were made by Irish physician Thomas Molyneux in 1695, who identified large antlers from Dardistown—which were apparently commonly unearthed in Ireland—as belonging to the elk (known as the moose in North America), concluding that it was once abundant on the island.

The original Blumenbach's description of Alce gigantea provides rather scant information about the species, specifying only that this particular kind of "fossil elk" comes from Ireland and is characterized by immense body size.

This particular feature mentioned by Blumenbach permitted to Roman Croitor to identify the type specimen of giant deer [11] that was figured and described for the first time in Louthiana of Thomas Wright.

[12] The holotype of Megaloceros giganteus (Blumenbach, 1799) is a well-preserved male skull with exceptionally large antlers found in Dunleer environs (County Louth, Ireland).

[16] The type and only species named in the description being Megaloceros antiquorum, based on Irish remains now considered to belong to M. giganteus, making the former a junior synonym.

[2] In 1828 Brookes published an expanded list in the form of a catalogue for an upcoming auction, which included the Latin phrase "Cornibus deciduis palmatis" as a description of the remains.

[2] Adrian Lister in 1987 judged that "the phase "Cornibus deciduis palmatis" constitutes a definition sufficient under the [International Code of Zoological Nomenclature] (article 12) to validate Megalocerus.

[22][23] The earliest possible record of the genus is a partial antler from the Early Pleistocene MN 17 (2.5–1.8 Ma) of Stavropol Krai in the North Caucasus of Russia, which were given the name of M. stavropolensis in 2016,[24] however this species has been subsequently suggested to belong to Arvernoceros.

[22] Jan van der Made has suggested that remains of an indeterminate Megaloceros species from the late Early Pleistocene (~1.2 Ma) of Libakos in Greece are closer to M. giganteus than the M. novocarthaginiensis-savini-matritensis lineage due to the shared molarisation of the lower fourth premolar (P4).

A large proportion of the known remains of M. giganteus are from Ireland, which mostly date to the Allerød oscillation near the end of the Late Pleistocene around 13,000 years ago.

[29] However, another study from the same year in the journal Nature utilising both fragmentary mitochondrial DNA and morphological data found that the Irish elk was indeed most closely related to Dama.

The skull and mandible of the Irish elk exhibit substantial thickening (pachyostosis), with the early and complete obliteration of cranial sutures.

In 1989, American palaeontologist Dale Guthrie suggested that, like bison, the hump allowed a higher hinging action of the front legs to increase stride length while running.

Irish elk do not appear have extended northward onto the open mammoth steppe in Siberia, rather keeping to the boreal steppe-woodland environments, which consisted of scattered spruce and pine, as well as low-lying herbs and shrubs including grasses, sedges, Ephedra, Artemisia and Chenopodiaceae.

[37] Irish elk antlers vary widely in form depending upon the habitat, such as a compact, upright shape in closed forest environments.

Geist, believing the Irish elk to have been a cursorial animal, concluded that a doe would have to have produced nutrient-rich milk so that her calf would have enough energy and stamina to keep up with the herd.

[37] The mesodont (meaning neither high (hypsodont) or low (brachydont) crowned) condition of the teeth suggests that the species was a mixed feeder, being able to both browse and graze.

[45] A stable isotope analysis of the terminal Pleistocene Irish population suggests a grass and forb based diet, supplemented by browsing during stressed periods.

[47] Comparisons of δ15N between Irish elk and red deer at the Middle Pleistocene site of Schöningen in Germany suggest that grasses were a more important component of the former's diet relative to the latter.

[48] Examination of histological sections of their long bones suggests that the species has relatively rapid growth rates, reaching skeletal maturity by around 6 years of age.

Analysis of the cementum layers of their teeth suggests that Irish elk reached a maximum lifespan of at least 19 years, comparable to moose.

After antler growing, stags could probably satisfy their nutritional requirements in productive sedge lands bordered by willow and birch forests.

Other sites probably resulting from exploitation of Irish elk by Neanderthals include Abri du Maras in southeast France, dating to 55-40,000 years ago.

[53] A mandible from Ofatinţi, Moldova dating to either the Eemian or the early Late Pleistocene, has been noted for having "tool-made notches on its lateral side".

[4] Several M. giganteus bones from the Chatelperronian levels of the Labeko Koba site in Spain are noted for bearing puncture marks, which have been interpreted as anthropogenic.

[56] A calcaneum from an associated lower hind limb from the early Holocene site of Sosnovy Tushamsky in Siberia is noted to have "two short and deep traces of cutting blows", which are interpreted as "clear evidence of butchery".

[4] Historically, its extinction has been attributed to the encumbering size of the antlers, a "maladaptation" making fleeing through forests especially difficult for males while being chased by human hunters,[13] or being too taxing nutritionally when the vegetation makeup shifted.

[4] A 2021 study found that M. giganteus saw a progressive decline in mitochondrial genome diversity beginning around 50,000 years ago, which accelerated during the LGM.

Lister and Stewart concluded in a study of the extinction of the Irish elk that "it seems clear that environmental factors, cumulatively over thousands of years, reduced giant deer populations to a highly vulnerable state.

"[4] Due to the abundance of Irish elk remains in Ireland, a thriving trade in their bones existed there during the 19th century to supply museums and collectors.