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The origin of the surname Costello provides a perfect illustration of the way the native Irish absorbed the invading Normans. Soon after the invasion, the de Angulo family, also known as "Nangle" settled in Connacht, where they rapidly became powerful. After only three generations, they had begun to give themselves a surname formed in the Irish manner, with the clan taking Jocelyn de Angulo as their eponymous forebear. Jocelyn was rendered Goisdealbh in Irish, and the surname adopted Mac Goisdealbhaigh, later given the phonetic English equivalent "Costello". Over many generations they carried on a bitter feud, in thoroughly Gaelic fashion, with their rivals in the region, the McDermotts. Their power continued up to the seventeenth century, centred in east Mayo, where they gave their name to the barony of Costello; the centre of their influence was the area around the modern town of Ballaghadereen. Today the surname is widely spread throughout Ireland, with the largest concentrations still in the historic homeland of Connacht.

In Donegal Costello has also been occasionally used as the anglicisation of Ó Casalaigh, more usually given as Cushley, and in Roscommon Casserley, from the original Mac Casarlaigh, has also at times been rendered as Costello.

In 1890 there were 142 births of the name, mainly concentrated in the four Atlantic counties of Limerick, Clare, Galway and Mayo.

John A. Costello (1891-1976) was Taoiseach (Prime Minister) of the Republic of Ireland from 1948 to 1951 and 1954 to 1957. He was responsible for finally taking the country out of the British Commonwealth in 1949. His son Declan Costello (1927 - ) was Attorney-General from 1973 to 1977 and was appointed President of the High Court in 1995.

Paul Costelloe (1945 - ) has acquired an international reputation as a fashion designer, and is particularly well-known for his use of Irish fabrics.

Elvis Costello (1950 - ) was born Declan McManus, the son of Irish parents who emigrated to London. He is one of the most original and influential songwriters of his generation.

Costello households by county in the Primary Valuation property survey of 1848-64.
Click on a county name for a parish breakdown(paying).
Antrim 3
Armagh 3
Belfast city 1
Carlow 3
Cavan 2
Clare 18
Cork 10
Cork city 3
Derry 6
Donegal 1
Down 3
Dublin 19
Dublin city 9
Fermanagh 5
Galway 194
Kerry 13
Kildare 13
Kilkenny 91
Laois 32
Leitrim 18
Limerick 39
Limerick city 11
Longford 9
Louth 3
Mayo 126
Meath 4
Monaghan 7
Offaly 19
Roscommon 14
Sligo 39
Tipperary 32
Tyrone 3
Waterford 8
Westmeath 19
Wexford 19
Wicklow 10