Carew is a Welsh language and Cornish Language habitation type surname; it has also been used as a synonym for the Irish patronymic Ó Corráin. Carey can be a variant.
The Cambro-Norman Carew family sprang from the same stock as the FitzGeralds: viz. from the union of Gerald de Windsor alias Gerald FitzWalter (1070–1136), the Norman Constable of Pembroke, Pembrokeshire and Nest ferch Rhys, Princess of Deheubarth, the 'Helen of Wales'. These Carews descend from Gerald and Nest's oldest son William FitzGerald de Carew. The family home was at Carew, Pembrokeshire, Welsh language 'Caeriw', from a fortified site and later castle, originally a holding of Nest's royal father, Rhys Ap Tewdwr. The usual derivation offered is that the root word is 'caer', Middle Welsh for 'fort'; the second element being possibly 'rhiw' – 'slope', or 'yw' – 'yew' (tree). The Pembrokeshire Coast National Park website has 'Caerau – fort (Locally pron Carey)'.
First, as will be shown below, not all modern Carews are of Carew, Pembrokeshire stock; some bear the name from cognate Cornish origins; and others as an Anglicised form, together with Carey, of the Irish patronymic Ó Corráin/Ó Carráin.
Secondly, some true Carews in Wales may have received their name in the variant form of 'Carey' or 'Cary', which is a traditional local pronunciation of the place Carew (see above), another version being 'Care-ew'. John Marius Wilson's 'Imperial Gazetteer of England and Wales' (1870–72) has 'Carew, or Carey, a village and a parish in the district and county of Pembroke'. Bannister (1871) writes that Carew is 'pronounced Car'-ew in Ireland; Car-ew' in Devon; Carey in Cornwall and Wales' (my italics). However, Carey in Britain generally is either from any one of at least six immigrant Gaelic-Irish patronymics Anglicised thus, or is from a Pre-Celtic or Celtic language river/habitation name in Somerset and Devon.
Gerald FitzWalter's second son Maurice FitzGerald, Lord of Lanstephan and grandson Raymond de Carreu, 'le Gros', took part, alongside Richard de Clare, 2nd Earl of Pembroke, or 'Strongbow', in the invasion of Ireland in 1171. The 'invasion' was almost a family affair, with many of the Cambro-Norman protagonists related through the matriarchal and 'polygamous' Nest: among the cousins of the Geraldines were Robert FitzStephen, Robert de Barry et al.