Irish moss, also known as Sagina, isn't really a moss at all. It is a low-growing groundcover with dense, lime-green leaves that form carpet-like mounds no more than two to four inches in height. In the summer, Irish moss blooms with tiny white flowers. Irish moss is commonly used in rock gardens and between paving stones; it grows best in well-drained, sandy soil that needs to be kept moist at all times. Growing Irish moss in sand isn't a stretch, then, as long as you make sure the sand doesn't dry out or become saturated.
Pour sand into plant beds, two inches lower than you want the the top of the bed to be. If your plot of ground is already composed of sand, clear away two inches from the surface, or allow for a tw-inch layer of soil mix to be added to the surface.
In a bucket or wheelbarrow, mix equal parts of sand and topsoil. Pour the mixture on top of the sand in the plant beds. If you're planting your Irish moss between paving stones or bricks, just pour in the 50-50 mix of potting soil and sand.
Plant tiny clumps of Irish moss by sticking your finger in the sand-soil mixture to a depth of about one inch and plugging the holes with the clumps. Plants should be put into the ground about 8 to 12 inches apart. If planting seeds, sprinkle the seeds on the sand-soil mixture and then cover them completely with a thin layer of sand.
Water gently and thoroughly. If you planted seeds, consider using a mister to prevent the seeds from being washed away.
Make sure the sand remains moist at all times. Don't let it dry out, even on the surface, but by the same token don't saturate it with water. Sand allows excellent drainage, but not if there's too much water, which could lead to root rot.