This Irish surname should be prefaced with the patronymic preposition "O" signifying "descendant of." The derivation is from the Gaelic O' Grianain, which itself derives from the word for sun - grian, plus the suffix "ain" a diminutive meaning "son of." However all is not so simple, because only some of the Irish Grennan's are pure Gaelic, many are of Norman-French extraction and descend from planters of the 12th century. In this case the derivation is from the Old French nickname "Gernon" meaning "The one with the moustache," dialectal transposition having turned it to Grennan. So which is which? In effect Grennans born in Co Mayo are believed to be Gaelic, whilst those born in Co Offally and further south are probably Norman French. There are in Co Limerick no less than five places called Ballygrennan, the place of the Grennans, and four Grenanstown's in Co Kerry, so it can be said that the clan whether Norman or Gaelic have left their mark. Amongst the records are those of Patrick and his wife Catherine, aged 24 and 19yrs, who left Ireland to escape the Great Famine, on the ship John R Skiddy of Liverpool on July 1st 1846 bound for New York, whilst earlier on May 3rd 1803 Andrew Greenan married Catherine Gun, at Clones, Co Monaghan. The first recorded spelling of the family name is shown to be that of Robert de Grenan, which was dated 1205, A land charter witness at Kildare, Co. Kildare, during the reign of King John of England, known as "Lackland" 1199 - 1216 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.