Barleria irritans is an upright or sub-erect shrublet, up to 400 mm high. The branches develop out of a woody rootstock and the stems are hairy on 2 sides (bifariously) or are slightly hairy when young, with long hairs on the nodes. The leaves are opposite, narrowly ovate, glabrous, with a few scattered hairs on the midvein beneath. The leaf margins are wavy, white in older leaves, spine-toothed; with a short petiole; the apex tapered into a sharp point.

The flowers are mauve, purple or blue, in a 1–8-flowered cluster. The corolla is 2-lipped, the upper lip 2-lobed, and lower lip 3-lobed. The corolla tube is long, hairy and cylindrical. The outer calyx lobes are elliptic-ovate, the margins are spine-toothed and the apex (tip) narrowed into a sharp point. There is one pair of bracteoles at the base of each flower and they are of an equal or unequal length, lanceolate or narrowly ovate, with 3 parallel veins, glabrous; the margins spine-toothed.

Fruit capsules are elliptic without a prominent beak and with 4 seeds per capsule. The ripe fruit capsules turn brown and open explosively when moistened.