Of two different prostrate plants of similar appearance, Sagina subulata is the more common. The other is Arenaria verna, usually called A. v. caespitosa. Both of these European natives make dense, compact, mosslike masses of slender leaves on slender stems. But A. verna has tiny white flowers in few-flowered clusters, while S. subulata bears flowers singly and differs in other details. In common usage, however, green forms of the two plants are called Irish moss, and golden green forms (A. v. 'Aurea' and S. s. 'Aurea') are called Scotch moss.
Both Sagina and Arenaria are grown primarily as ground covers for limited areas; they're also useful for filling gaps between paving blocks. They won't grow well under conditions that suit true mosses. They need good soil, good drainage, and occasional feeding with controlled-release fertilizer. They take some foot traffic and tend to hump up over time; control this by occasionally cutting out narrow strips, then pressing or rolling lightly. Set out plants 6 inches apart for fast cover. Control snails, slugs, and cutworms. Can tend to 'melt away' during long stretches of hot, humid weather.