Asplenium onopteris, commonly known as Irish spleenwort or western black spleenwort, is a perennial herbaceous fern native to rocky slopes and crevices within Mediterranean woodlands, scrublands, and coastal cliffs, as well as the Eastern Atlantic islands. It is diploid and serves as one of the progenitor species of the tetraploid A. adiantum-nigrum. First described by Linnaeus in 1753, it is a member of the "Onopteris subclade" within the "Pleurosorus clade." This fern typically forms dense tufts of triangular, pinnate fronds that are dark green in color, reaching up to 20 cm in length. The sori, or spore-producing structures, are found on the underside of the fronds and are covered by a membranous indusium.

Irish spleenwort is valued for its hardiness and ability to thrive in shaded, rocky environments, making it a suitable choice for rock gardens, green roofs, and as a ground cover in shaded areas. It requires well-drained soil, moderate moisture, and can tolerate a range of light conditions from part shade to full shade. While it is not prone to serious diseases or pests, it can suffer from root rot if overwatered or planted in poorly drained soils. In cultivation, it is appreciated for its low maintenance and textural contrast in fern collections or woodland gardens.CC BY-SA 4.0