This interesting surname has two distinct possible sources; firstly, it may be of Old French origin, introduced into England after the Conquest of 1066, and a locational name from any of several places in Calvados called Oilly, with the fused preposition "de". The placename derives from the Gallo-Roman personal name "Ollius", with the local suffix "-acum". Locational surnames were originally given as a means of identification to those who left their village or place of origin to settle elsewhere. Henry Dayly is noted in the Hundred Rolls of Oxfordshire (1279). The surname may also be of Irish origin, and an Anglicized form of the Gaelic "O Dalaigh", composed of the elements "O", male descendant of , with "Dalach", a personal name from "dal", meeting, assembly. The "O Dalaigh" ancestry goes back to the 4th Century, to Niall of the Nine Hostages, the High King who had his palace at Tara, Co. Meath. They are very prominent in records dating from the 12th to the 18th Centuries, when their extraordinary genius for bardic literature was manifest in the bardic school set up by Cuconnacht O Dalaigh in Westmeath. From the 11th to the 15th Centuries they were hereditary poets and minstrels to most of the leading families. On July 20th 1701, James, son of James and Susanna Dailey, was christened at St. Mary Whitchapel, Stepney, London, and Samuel, son of John Dailey, was christened on June 24th 1757, at Clones, Co. Monaghan, Ireland. The first recorded spelling of the family name is shown to be that of Robert de Oilgi, which was dated 1086, in the Domesday Book, during the reign of King William 1, known as "The Conqueror", 1066 - 1087. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.