'Shamrock' term used to describe many types of clover
The English word 'shamrock' is believed to be a corruption of the two Irish words 'seamair' and 'óg'. 'Seamair' is the Irish for clover and as everybody knows 'óg' means young. So, as Gaeilge, 'seamróg' is a blanket term for young members of the clover group of flowering plants.
The term 'seamróg' is also used for the smaller-leaved members of the clover group in the sense that while they may not be young in age they look young because of their small leaves.
The clover flora of Ireland comprises fifteen different species. Black Medick is so closely related to clovers that it is also known in Irish as Seamróg but it is in a different genus or group of its own.
Black Medick is often confused with clovers and is separated from them by two particular traits: medicks have strongly-curved or spirally-twisted seed pods whereas all clovers have straight seed pods and medicks have a point at the tip of each leaf whereas in clovers the midrib of the leaf does not end in a point as shown in the image above.
Clovers and medicks are both members of the large pea or legume family that includes such diverse members as trees like the Laburnum, shrubs like Broom and Gorse, and herbaceous plants like lupins, vetches and other trefoils.
In the past, local tradition in different parts of Ireland recognised different members of the clover group as shamrock. Black Medick was also worn but the most popular candidate for the real shamrock title is the native Lesser Trefoil (Trifolium dubium) a small-leaved, yellow-flowered clover that is a very common in pastures, lawns and waste places. Lesser Trefoil is also the species that is grown commercially and sold as shamrock.
The concept of leaflets is not immediately obvious. Some plants have their leaves split into parts called leaflets. Ash tree leaves have 9-15 leaflets arranged like a fern frond. The Horse-chestnut has 5-7 leaflets splayed out like fingers on an open hand. And clovers all have the three leaflet arrangement so familiar to everybody in shamrock.
Shamrock is widely regarded as one of the symbols of Ireland and is closely associated with Saint Patrick who is said to have used the fact that shamrock has one leaf divided into three leaflets as a metaphor for the Christian mystery of the Holy Trinity, one God in three persons: the Father, the Son and the Holy Spirit.