Desert plants, though drought tolerant by nature may benefit from water during severe droughts. Plants kept indoors or under an impermeable cover need regular, though very infrequent watering. Irrigation systems for desert plants are designed to provide them with very infrequent watering. The few times desert plants are watered, the amount must be abundant enough to recharge the plant's own water reserve system until the next deluge.
Drip lines and soaker hoses are the best ways to go if you have only a few plants, or a small, manageable area to grow them in. Drip lines are networks of pressurized hoses, usually metal, that have valves to control the water flow at a low (usually drip) rate. They can be set to timers so that you don't overdo end up with desert plant sitting in a bog, and individual drips can be set into a pot. Soaker hoses can be turned on to gradually flood an area infrequently, but aren't helpful for potted plants.
While you can still use drip irrigation or soaker hoses on a large area of desert plants, the initial cost and maintenance of such a system can quickly become time and cost prohibitive. Instead, canals should be dug to catch and direct water through the desert garden. Canals are useful on relatively flat land, where either Mother Nature or the gardener can let loose a rare torrent of water that gradually sinks into the ground.
Swales are ditches that are dug along the contour of the land, so that the entire ditch is at the exact same elevation. When it rains, water is caught in the swale instead of washing down the incline and out of the garden or farm. (See reference 3) The water in the swale drains slowly underground, recharging water reserves and providing a deep water resource for desert plants with taproots designed to take advantage of them.
No Irrigation System
Desert plants have the benefit of being highly drought resistant. Unless you live in a desert, the chances of desert plants needing water so frequently that you need to invest in an irrigation system is minimal. Instead, water only during the severest droughts, or water only the plants that can't hold out until the next rainstorm. More desert plants die from over-indulgence with water than from lack of it.
If desert plants are interspersed with plants requiring more frequent water, the situation gets tricky. Plant the desert plants on a mound so that moisture drains away quickly to the surrounding plants that can tolerate extra moisture. Avoid an irrigation system that constantly waters from overhead, like sprinklers. This will cause many desert plants to rot. When watering by hand, water only at the base of the plant, never over the top growth.