Asplenium onopteris is known as the Irish spleenwort or western black spleenwort. It is difficult to identify compared with the black spleenwort, Asplenium adiantum-nigrum. The main difference is that A. onopteris is diploid and is one of the two parents of the tetraploid A. adiantum-nigrum (the other being the diploid A. cuneifolium). Armed with a microscope, the most consistent observable difference between A. onopteris and A. adiantum-nigrum is that A. onopteris spores have a mean diameter of 28 μm and are almost all smaller than 31 μm, whereas those of A. adiantum-nigrum have a mean diameter of 34 μm and are almost all larger than 31 μm. The leaflets of typical A. onopteris are narrower in relation to their length than those of typical A. adiantum-nigrum, but this is not a reliable means of identification.
Asplenium onopteris is found all around the Mediterranean Sea, including North Africa. There are isolated findings as far north as Ireland and Poland.