Unlike some widespread members of the genus, B. irwinii has a relatively restricted geographic distribution, being found only in cool, hypermaritime regions where it grows primarily on conifer trunks and branches, especially those of Pinus species in well-ventilated locations.

[2] Molecular phylogenetics analysis based on ITS and MCM7 (genetic markers) places B. irwinii within section Bryoria clade 2, where it forms a strongly supported group with several related species including B. furcellata, B. nitidula, B. trichodes, B. araucana, B. nadvornikiana, B. simplicior and B. poeltii.

The surface of the lichen has a slight shine and is decorated with numerous small, pale brownish spots (called pseudocyphellae) that are fusiform or spindle-shaped.

[2] Bryoria irwinii is endemic to the Pacific Northwest region of North America, with a restricted geographic range extending from central British Columbia northward to southeastern Alaska.

The species' restricted distribution and specific habitat preferences suggest it has evolved to grow in the distinctive climate of the Pacific Northwest coast, where cool temperatures and high humidity are common year-round.