The first alcohol consumed in Ireland was beer (5000 years ago) and mead, however, surprisingly alcohol was not considered the most important drink. That honour went to milk!
While alcohol may not have been that important to the early Irish, the oldest pub in Ireland, 'The Brazen Head' in Dublin, has been dated to 1198AD.
However, the early Irish were besotted with cattle, and hence milk. The story of the Táin Bó Cúailnge or Cattle Raid of Cooley is the national epic. It is centred around the power struggle between Queen Medb and her husband King Ailill and the main unit of currency were cattle!
Any ceremonies of importance were cemented with drinking milk. Hospitality was considered a duty related to honour. Refusing the gift of a drink from your host was therefor a hostile act, and was calculated to give huge offence. In a curious echo down the centuries, the caricature of Mrs. Doyle in the television show Father. Ted is a strangely close nod to the historical importance of hospitality. Milk was the most important ceremonial drink until approximately the 17th century, when tea and alcohol took over its social significance.