Zirku is of a similar geological structure and appearance to the island of Arzanah, 50 kilometres to the southwest. Qarnein, its nearest neighbour, lies 20 kilometres to the northwest. The island, with its central hilly core rising to 160 metres, covers around 10 square kilometres. In recent years, it has been transformed by the oil industry.
Wildlife has suffered from the depredations of introduced cats and rats, although, as on Arzanah, these are now subject to a control programme. Colonies of Socotra cormorants (Phalacrocorax nigrogularis) and sooty gulls (Larus hemprichii), and perhaps other seabirds, have disappeared, although an important colony of red-billed tropic-birds (Phaethon aethereus) albeit much reduced in size, still survives in scree below the cliffs in the northeast of the island. Passage falcons are sometimes trapped for hunting purposes.
Turtles, most probably hawksbill, are reported to come ashore to nest.
Zirku first appears in history in a list of islands names published by the Venetian jeweller Gasparo Balbi in 1580, although recent development has removed virtually all evidence of earlier occupation.