Ask three people what Irish moss is and you could get three answers: a purplish seaweed off the Atlantic coast, a green cushion sold in garden centres, or the "sea moss" gel filling health-food shelves. They are quite different living things, and not one of them is a moss.
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The seaweed: carrageen
The oldest thing to carry the name is Chondrus crispus, a red alga of rocky Atlantic shores, Ireland and Britain among them. In Ireland it has long been known as carrageen, from a word meaning "little rock". It grows in flat, fan-shaped fronds that fork again and again, dark red to purple, sometimes flashing blue-green underwater and drying to a horny yellow. Simmered in milk it gives up a setting agent and stiffens into a soft jelly, the old basis of blancmanges and milk puddings, and in harder years a food that helped carry families through the Irish famine. That gelling substance, carrageenan, is still drawn from it on an industrial scale and thickens a surprising amount of what sits on supermarket shelves.
Sea moss, the modern label
Sea moss is the same notion under a newer name, though usually a different seaweed. Much of what is sold as sea moss, whether as a ready-made gel or as dried golden tufts, is not Chondrus but tropical red algae such as Eucheuma and Gracilaria, farmed in warm shallow water. It is promoted for a long list of supposed health benefits on the strength of its minerals and fibre. Those claims are worth treating coolly: seaweed is a decent food, but the bigger promises made for sea moss are thinly supported, and its iodine is a real consideration for anyone with a thyroid condition. None of this is medical advice, and a word with a doctor is sensible before you lean on any supplement. The only point that matters here is that sea moss, like carrageen, is a seaweed, and not a moss in the botanical sense at all.
The garden plant
In a nursery, Irish moss means something else entirely. There it is Sagina subulata, a low flowering plant of the pink family that mats into a fine green cushion looking very much like a moss. Its yellow-leaved form sits on the same bench labelled Scotch moss. Both are grown as a soft groundcover and lawn substitute, and in early summer both scatter themselves with minute white flowers that no moss could ever produce. To muddy things further, a similar-looking cushion sold under the very same names is sometimes a pearlwort or an Arenaria rather than Sagina, but the conclusion is unchanged: these are true flowering plants, with proper roots and the internal plumbing that mosses do without.
Telling them apart
None of the three is hard to place once you know where to look. A moss bears neither flowers nor roots; it clings by hair-fine rhizoids and travels by spores. The garden "Irish moss" flowers, and lifts out of the ground on a genuine root system. The seaweeds live in salt water and own no leaves, stems or roots at all, only a flat frond and a holdfast gripping the rock. Their shared name rests on nothing more than a shared look, low and soft and green, or the woolly moss-like tangle of dried carrageen. It is the same loose turn of speech that hangs the word on reindeer moss, which is a lichen, and on the marimo, which is a ball of alga.
Growing Sagina in the garden
If it is that green cushion you actually want, Sagina makes a pleasant, walkable carpet for the gaps between paving or the edge of a small path, though it asks rather more than a real moss does. Give it moist but free-draining soil and a spot in sun to light shade; it resents both drought and standing wet, and in a hot dry spell it scorches off in brown patches and humps up into little mounds that want trimming flat again. Think of it as a slightly fussy perennial rather than a plant-and-forget moss. For a lawn that truly is moss, which wants quite different ground and treatment, there is making a moss lawn; and moss as ground cover weighs up the low green options more widely.