The Killarney fern (Vandenboschia speciosa) is the only species of its genus in the European flora and therefore not easily confused.
The largest of the three European filmy-ferns, it is easily told apart from the two members of the genus Hymenophyllum by its larger size, with fronds which can reach more than 30cm long (those of the Hymenophyllums are usually <10cm).
In all of the members of this rather ancient but highly specialised family the fronds of the sporophyte, or spore bearing generation are very delicate and translucent in appearance, being only a single cell thick over much of their surface. They are, however, surprisingly tough.
The fronds with their winged stalks are borne at intervals along a hairy, creeping rhizome, which is thicker (>1mm) than those of the Hymenophyllums and not thread-like. This adheres to its usually rocky substrate but produces occasional thick roots with which it anchors itself more firmly.
Another distinguishing feature, if you are lucky enough to find them because many British colonies have never been seen to produce them, are the rather cigar shaped tubular sporing structures borne at the ends of the leaflets. These have the sessile sporangia, tightly spirally arranged around a central bristle like receptacle. This continues to elongate, projecting from the tubular receptacle, until desiccation leads to sporangial dispersal. The resulting structures give the plant another of its common names, “Bristle fern”.