Thallus white or pale grey, occasionally light blue-grey, areoles arising on prothallus when visible, soon coalescing and cracking; ± plane or gently convex between cracks, 100–400 µm thick; very frequently with raised rugose galls caused by Cecidonia xenophana. Prothallus blue-black to black.
Anamorph: conidiomata pycnidia, frequent, to 600 µm diam., multilocular, with several ostioles, the apex irregular, black. Conidia 8–11 x ca 1 µm, aseptate, colourless, straight.
Teleomorph: ascomata apothecia, to 2 mm diam., becoming sessile as soon as the young disc begins to expand, margin smooth or rarely faintly striate, 60–220 µm wide, eventually ± excluded in old apothecia; disc plane to strongly convex when mature; pruina sometimes present on inner edge of young apothecial margin, present or absent on mature disc. Exciple densely pigmented at surface, brown to dull green-brown or dark dull greenish blue, within paler, brown throughout. Hypothecium brown to red-brown. Hymenium 100–140 µm high. Epihymenium dull green-brown or brown, K–. Interascal tissue of paraphyses, 1.5–2.0 mm diam., the apices 3-3.5 µm diam., the cells colourless or dilute brown, surrounded by pigment (epihymenial pigment mostly extracellular). Asci clavate, 8-spored, tholus with an I+ blue tube structure. Ascospores (15.0–) 16.5–20.5 (–21.5) x (7.5–) 8.0–10.5 (–12.5) µm, ellipsoidal, aseptate, colourless, with a gelatinous sheath.
Chemistry: thallus K–, PD–, C– ; microscopic preparations in K not exuding minute droplets.
Not formally evaluated. The species appears to be common in Wales and is probably under-recorded elsewhere in GB&I due to confusion with P. contraponenda. There are no known significant conservation threats to the species itself.
Phylogenetically related to Porpidia cinereoatra and P. contraponenda. P. irrigua differs from P. contraponenda in the generally thinner thallus (although the ranges of thickness overlap greatly), the apothecia which are sessile from a very early stage, and in chemistry. In P. contraponenda the apothecia are at least partly immersed in the thallus.
In GB&I, reported from England (VC69 Westmorland, VC70 Cumberland), Wales (VC41 Glamorgan, VC42 Breconshire, VC43 Radnorshire, VC45 Pembrokeshire, VC48 Merioneth and VC49 Caernarvonshire), Scotland (VC88 Mid-Perthshire) and Ireland (VCH1 S Kerry).
On damp siliceous rock, frequently where seasonally flushed.