Miragaia is based on holotype ML 433, a nearly complete anterior half of a skeleton with partial skull (the first cranial material for a European stegosaurid).

[2] Among the recovered bones were most of the snout, a right postorbital, both angulars of the lower jaws, fifteen neck vertebrae (the first two, which articulated with the skull, were absent), two anterior dorsal vertebrae, twelve ribs, a chevron, the shoulder bones, most of the forelimbs including a possible os carpi intermedium, a right first metacarpal and three first phalanges; and thirteen bony plates plus a spike.

[1] The bones were not articulated but dispersed over a surface of about five to seven metres, though there was a partial concentration of fossils that could be salvaged within a single block.

Casts were made of the holotype bones and partially based on these a life-size skeletal model was constructed from polyurethane and polyester resin parts.

[2] In 2010, Alberto Cobos and colleagues noted that all the diagnostic characters of Miragaia longicollum are based on skeletal elements that are absent in the Dacentrurus holotype found in England in layers of about the same age, while all traits that can be compared are shared by both genera.

Therefore, they proposed that Miragaia is a junior synonym of Dacentrurus, meaning that it is the same dinosaur, because it is not possible to differentiate the two taxa through their holotypes.

[3] Francisco Costa and Mateus considered Miragaia longicollum as a valid taxon, describing a newly recognised specimen, MG 4863, that had already been excavated in 1959 by Georges Zbyszewski but was only prepared between 2015 and 2017.

On the neck two rows of triangular bony plates are present that have a lightly convex outer side and a notch at the upper front edge creating a hook.

The Thyreophora, the larger group they belong to, originally seem to have had nine neck vertebrae and this is also the number shown by the basal stegosaurian Huayangosaurus.

The possible food gathering function of the neck makes sexual selection a less plausible explanation, but is not in itself entirely convincing: though the contemporary Iberian sauropods Lusotitan, Dinheirosaurus and Turiasaurus were all very large and might not have competed with a medium-height browser, the niche partitioning is still problematic because in Iberia stegosaurian remains have been referred to Dacentrurus and Stegosaurus, which would have possessed a feeding envelope or feeding height stratification overlapping that of Miragaia.

There is some evidence for increasing vertebral length in Miragaia and Stegosaurus, compared to more basal species, but this is equivocal and could be due to post-mortem distortion; this mechanism was seen as a minor factor in neck elongation.

The neural spines of the rear cervicals and front dorsals are transversely expanded at their upper ends due to rugosities serving as an attachment for tendons.

The tuberosity of the rear humerus serving as attachment for the musculus triceps brachii is well-developed but the vertical ridge running from it to below is not.

In the pelvis, the pubic bone had a deep front part, the processus praepubicus, with a little upward projecting process as seen in Dacentrurus; the rear shaft had a lightly expanded tip.

[1] A rather long, narrow and straight preserved spike was at first considered to have been a shoulder spine, but was later seen as part of some tail arrangement.

[1] In 2017, Raven and Maidment published a new phylogenetic analysis, including almost every known stegosaurian genus:[11] Lesothosaurus diagnosticus Laquintasaura venezuelae Scutellosaurus lawleri Emausaurus ernsti Scelidosaurus harrisonii Alcovasaurus longispinus Sauropelta edwardsi Gastonia burgei Euoplocephalus tutus Huayangosaurus taibaii Chungkingosaurus jiangbeiensis Tuojiangosaurus multispinus Paranthodon africanus Jiangjunosaurus junggarensis Gigantspinosaurus sichuanensis Kentrosaurus aethiopicus Dacentrurus armatus Loricatosaurus priscus Hesperosaurus mjosi Miragaia longicollum Stegosaurus stenops Wuerhosaurus homheni The authors stressed that the only synapomorphy, shared derived trait, supporting the Dacentrurus-Stegosaurus clade was the possession of the long cervical postzygapophyses, and that these are in fact unknown for Dacentrurus itself, so that its close position to Stegosaurus was merely based on the new data provided by the description of Miragaia.