Cider is called seidr in Welsh.
Although alcoholic drinks have been made form apples in Britain since at least Roman times, cider-making as we know it today came over the channel with the Normans at around the 12th Century. It had swept across the border from Herefordshire into Wales by about the 14th Century. It became well established in the farming communities of the south-eastern part of Wales and in the region of mid-Wales adjacent to the English border, but the harsh mountains prevented its spread much further. The shelter of the Usk and Wye valleys allowed orcharding to reach inland areas such as Brecon and Builth Wells. The soils and climate of South East Wales is particularly favourable for apple growth and at one time almost every large farm had its own cider mill as well as orchards. Welsh varieties of apples and pears are often distinct from those grown in England, giving cider from Wales a flavour noticeably different from ciders from nearby regions.
In common with the cider-making areas of England, cider in Wales went into decline with the industrialisation of agriculture just after the end of World War II. With this, small-hold production of cider made on farms as a beverage for labourers died out. However, unlike in England, no commercial Welsh cider-makers were left to continue the tradition. But the orchards remained, and in the 1980s the first of a new wave of craft cider-makers started to use the fruit from the old trees and many new orchards have been planted to revive the tradition of cider-making in Wales.
More information can be found in the article by Gareth Beech, senior curator at the National Museum of Wales: Beech, Gareth. An Introduction to Cider Making in Wales. Melin 23 (2007): 50-66.