The name comes from the Ancient Greek word εἰρήνη (iren) meaning "peace", in reference to the end of World War I.

Although it has been generally placed in the tribe Phyllotini, genetic evidence does not support any close relationships with other genera, so that it is now classified as a member of the subfamily Sigmodontinae incertae sedis (of uncertain position).

[5] The group of genera related to Phyllotis was later formalized as the tribe Phyllotini, and Irenomys was often included there, but also excluded from it by some authors.

[9] In 1995, a cladistic analysis of Phyllotini on the basis of morphology provided evidence in favor of placement of Irenomys in the group, with some support for a close relation to Andinomys.

[10] From 1999 on, DNA sequence data cast doubt on this assignment, as studies using the mitochondrial cytochrome b gene and the nuclear IRBP gene placed Irenomys in a variety of positions, all outside Phyllotini, with Scolomys, Sigmodon, Euneomys, and various large clades of sigmodontines all as sister groups in some analyses.

[2] The upperparts are rufous with fine dark lines and the underparts are buff, with the exact color varying by subspecies.

The tail, which ends in a slight pencil, is dark brown, with a somewhat lighter area present on the ventral side in some individuals.

The molars are strongly hypsodont (high-crowned) and consist of transverse, diamond-shaped laminae (plates), separated by deep valleys, which are joined at the midline by narrow ridges, similar to those of the African elephant.

[2] It generally occurs in humid and densely forested habitats, often with bamboo vegetations, but a specimen has been reported from riparian vegetation at a small stream near the southern limit of its distribution and it is also found in unforested steppe habitat with scattered Austrocedrus chilensis trees.