The shamrock (Irish seamróg) is a plant with trifoliate leaves and it is a symbol of St. Patrick's Day because tradition has it that St. Patrick used the plant to explain the theological doctrine of the Holy Trinity (three persons in one God). The word is a diminutive of Irish seamar 'clover'. The ancient Druids associated the shamrock with the coming of spring and the rebirth of the natural world at the vernal equinox. The shamrock was later adopted as the national symbol of Ireland. The phrase "to drown the shamrock" means 'to go drinking on St. Patrick's Day'. Clover is the common name for this species of trefoil, but it was spelled various ways prior to 1600, which is also what happened to shamrock until around 1577.

The alcoholic liquor called beer has been around for quite some time. Before 6000 BC, beer was made from barley in Sumeria and Babylonia. Reliefs on Egyptian tombs dating from 2400 BC show that barley or partly germinated barley was crushed, mixed with water, and dried into cakes. When broken up and mixed with water, the cakes give an extract that was fermented by microorganisms accumulated on the surfaces of fermenting vessels. The word's etymology is uncertain but it came to us through West Germanic (bier) and is based on Latin biber 'drink' from bibere 'to drink'.

In Irish folklore, a leprechaun was a tiny sprite or fairy who carried a purse containing a shilling. The word is derived from Old Irish luchorpán 'wee ones', from lu 'small' + corp 'body'. Over the years, the word luchorpán was confused with an Irish word meaning 'one shoemaker'. The leprechaun started being depicted as a solitary creature working on a single shoe instead of a pair.

Ireland's (Irish Éire) first human occupation did not begin until a late stage in the prehistory of Europe. It has generally been held that the first arrivals were Mesolithic hunter-fisher people, represented largely by flintwork found mainly in ancient beaches in the historic counties of Antrim, Down, Louth, and Dublin. The word Éire may itself mean 'western land', from a root word related to Gaelic iar 'west'. It first appears as Ierne in Greek writings which may have a base dating as early as the 5th century BC. The name appears as Iouernia in Ptolemy's map (c. AD 150) and has also been found translated into Latin as Iuverna. The Latin form, Hibernia, appears in the works of Caesar, who may have confused it with the Latin word hibernus 'wintry'. Ériu was an Old Irish form of Éire, and was seen in the earliest of Irish literature.

Rainbow comes from Old Norse regnbogi (becoming Old English renboga, ren 'rain' + boga 'bend, bow') and is a bow or arch of the colors of the prism that is formed in the sky opposite to the sun by the reflection, double refraction, and dispersion of the sun's rays in falling drops of rain. There are many stories about the pot of gold existing at the end of a rainbow.

St. Patrick, the patron saint of Ireland, was not actually Irish. He was born around AD 385, somewhere in Roman Britain, possibly near Dumbarton, Scotland. At 16, he was captured by Irish raiders looking for slaves and he was taken there to tend sheep. After six years of slavery, he ran away and ended up wandering through southern Gaul (France) and Italy. There, he had a vision from God which told him to return to Ireland and convert the pagans to Christianity. Returning to Ireland around 432, St. Patrick did missionary work until he died on March 17 in 464. The country of Ireland went into mourning. The first St. Patrick's Day celebration in the United States was in 1737 in Boston.