This species was treated as the most widespread and morphologically variable species of Danais by Puff & Buchner (1994), with a range in Mauritius, Reunion, and throughout Madagascar and morphological variation in particular in leaf size and shape and inflorescence size. Puff & Buchner (1994) characterized this species by its small triangular stipules, small to rather large leaves that are usually quite stiff to leathery in texture, rather short cymose inflorescences with the flowers and fruits all pedicellate, calyx limb with a short tubular portion and broadly triangular lobes generally about as long as the tubular portion, rather small corollas, and medium-sized fruits.
Danais was subsequently studied with molecular data by Razafimandimbison et al. (2012), who found the plants of Mauritus and those of Madagascar separated on two distinct lineages in their phylogram. They included all of these plants in this analysis as "Danais fragrans", but noted in their discussion comments that the Malagasy plants seem distinct and probably should be separated and called Danais lyallii. They later adopted that name in another analysis (Razafimandimbison et al., 2022), and the formal taxonomic statement separating these species and clarifying the identities of associated names was published by Taylor et al. (2024).
The plants from Madagscar included by Puff & Buchner (1994) in Danais fragrans along with a number of the plants they identified as Danais cernua were included by Taylor et al. in Danais lyallii, which was also separated somewhat differently from Danais cernua than done by Puff & Buchner. All three species, the two in Madagascar and the Mascarene Danais fragrans, are quite similar and sometimes subtle to distinguish.