Usually sold in flattened rounds, it is often served toasted with butter along with a cup of tea in the afternoon. The dough is sweeter than sandwich bread, but not as rich as cake, and the sultanas and raisins add flavour and texture to the final product. In Ireland it is sometimes called Bairín Breac, and the term is also used as two words in its more common version. This may be from the Irish word bairín - a loaf - and breac - speckled (due to the raisins in it), hence it means a speckled loaf (a similar etymology to the Welsh bara brith).
Other source: Barm is the technical term for the yeast filtered out of beer in the final production stages. It was a cheaper yeast source than commercial yeast used for making breac (speckled loaf).
Barmbrack is the centre of an Irish Halloween custom. The Halloween Brack traditionally contained various objects baked into the bread and was used as a sort of 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, 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 wed within the year. Other articles added to the brack include a medallion, usually of the Virgin Mary to symbolise going into the priesthood or to the Nuns, although this tradition is not widely continued in the present day.
Commercially produced barmbracks for the Halloween market still include a toy ring.
Reference to barmbracks is made in Dubliners by James Joyce. The following example can be found in the first paragraph of Joyce's short story Clay:
The fire was nice and bright and on one of the side-tables were four very big barmbracks. These barmbracks seemed uncut; but if you went closer you would see that they had been cut into long thick even slices and were ready to be handed round at tea.