Halloween is a significant cultural event in Ireland; it is widely celebrated. It is known in Irish as Oíche Shamhna (Irish pronunciation: [ˈiːhə haunˠə] ee-hah how-nah), literally "Samhain Night." In the Irish language, Samhain is the name for the month of November. Pre-Christian Celts had an autumn festival, Samhain (Irish pronunciation: [ˈsˠaunʲ], from the Old Irish ˈˈsamainˈˈ), "End of Summer," a pastoral and agricultural "fire festival" or feast, when the dead revisited the mortal world and large communal bonfires would hence be lit to ward off evil spirits. Costumes and masks being worn at Halloween goes back to the Celtic traditions of attempting to copy the evil spirits or placate them. Samhnag — Candle lanterns carved from turnips, were part of the traditional festival. Large turnips were hollowed out, carved with faces and placed in windows, also used to ward off harmful spirits.

Pope Gregory IV standardized the date of All Saints' Day, or All Hallows' Day, on November 1 in the name of the entire Western Church in 837. As the Church day began at sunset, the holiday coincided exactly with Samhain. It is claimed that the choice of date was consistent with the common practice of leaving pagan festivals and buildings intact (e.g., the Pantheon) while overlaying a Christian meaning, however no reliable documentation indicates such a motivation in this case. While the Celts might have been content to move All Saints' Day from their own previous date of April 20, the rest of the world celebrating it on May 13, it is speculated without evidence that they were unwilling to give up their preexisting autumn festival of the dead and continued to celebrate Samhain.

On Halloween night, adults and children dress up as creatures from the underworld (e.g., ghosts, ghouls, zombies, witches, and goblins), light bonfires, and enjoy spectacular fireworks displays – in particular, the city of Derry is home to the largest organized Halloween celebration on the island, in the form of a street carnival and fireworks display. It is also common for (illegal) fireworks to be set off by members of the public for the entire month preceding Halloween as well as a few days after. Halloween was perceived as the night during which the division between the world of the living and the otherworld was blurred so that spirits of the dead and inhabitants from the underworld were able to walk free on the earth. It was believed necessary to dress as a spirit or otherworldly creature when venturing outdoors to blend in, and this is where dressing in such a manner for Halloween comes from. This gradually evolved into trick-or-treating, because children would knock on their neighbours' doors in order to gather fruit, nuts, and sweets for the Halloween festival. Salt was once sprinkled in the hair of the children to protect against evil spirits.[citation needed]

Houses are frequently adorned with pumpkins, or traditional turnip carved into scary faces; lights or candles are sometimes placed inside the carvings, resulting in an eerie effect. The traditional Halloween cake in Ireland is the barmbrack, which is a fruit bread. The Halloween Brack traditionally contained various objects baked into the bread and was used as a fortune-telling game. In the barmbrack were a pea, a stick, a piece of cloth, a small coin (originally a silver sixpence), and a ring. Each item, when received in the slice, was supposed to carry a meaning to the person concerned: the pea, the person would not marry that year; the stick, "to beat one's wife with," would have an unhappy marriage or continually be in disputes; the cloth or rag, would have bad luck or be poor; the coin, would enjoy good fortune or be rich; and the ring, would be married within the year. Commercially produced barmbracks for the Halloween market still include a toy ring.[citation needed]

Games are often played, such as bobbing for apples, in which apples, peanuts, and other nuts and fruit and some small coins are placed in a basin of water. The apples and nuts float, but the coins, which sink, are harder to catch. Everyone takes turns catching as many items possible using only their mouths. In some households, the coins are embedded in the fruit for the children to "earn" as they catch each apple. Another common game involves the hands-free eating of an apple hung on a string attached to the ceiling. Games of divination are also played at Halloween, but are becoming less popular.

Lunchtime (the midday meal, sometimes called "dinner" in Ireland), on Halloween is called Colcannon.

Halloween celebration in France began in 1997 on behalf of retail companies, but has not caught on in any wide sense. Popularity peaked in 2000, but declined after that. In 2006, many French newspapers wrote on the death of Halloween. It was purely commercial, and overlapped with the Toussaint day (Catholic festival) that takes place November 1.[citation needed]

Scotland, having a shared Gaelic culture with Ireland, has celebrated the festival of Samhain (pronounced Sow-win) robustly for many centuries. The autumn festival is pre-Christian Celtic in origin, and is known in Scottish Gaelic as Oidhche Shamhna, the "Summer's night." During the fire festival, souls of the dead wander the Earth and are free to return to the mortal world until dawn. Traditionally, bonfires and lanterns (samhnag in Scottish Gaelic) would be lit to ward off the phantoms and evil spirits that emerge at midnight. The term Samhainn or Samhuinn is used for the harvest feast, and an t-Samhain is used for the entire month of November.

As in Ireland, the exact customs involved with celebrating Halloween from ancient times to pre-industrialised Scotland are lost and lack primary documentation to distinguish the ancient customs from the modern counterpart. The Witchcraft Act of 1735 was repealed in the 1950s. The name Halloween is first attested in the 16th century as a Scottish shortening of the fuller All-Hallows-Even, that is, the night before All Hallows Day. Scotland's National Bard, Robert Burns, portrayed some of the customs in his poem "Hallowe'en" (1785).

Halloween was seen as the time when the division between the world of the living and the otherworld was blurred. Many of the traditional customs derive from ancient divination practices and ways of trying to predict the future. By the 18th century, most of the customs were methods for young people to search for their future husbands or wives. As Samhainn was originally a harvest festival, many of these practices are connected with food or the harvest and fertility. One old custom associated with the Western Isles was to put two large nuts in the hearth of a peat fire to represent the person and his or her intended spouse. If the nuts curled together when they warmed up, it was deemed to be a good omen; but if they jumped apart, then it was time to look for another sweetheart.[citation needed] Islanders from Lewis traditionally poured ale into the sea as a libation to a marine God called "Seonaidh," or "Shoney," on Celtic Samhain or Halloween so that he would send seaweed to the shore to fertilise the fields for the coming year. Seonadh in Scottish Gaelic means sorcery, augury, or Druidism, and it is possible that the custom of Shonaidh is the direct link to an ancient form of Celtic god worship that has been Christianised.[citation needed] As "Seonaidh," which is Gaelic "Johnny," it may also be a reference to one of St. John and an invocation of him.