The barony of Costello in County Mayo, takes its name from this family which was established there. The Costello families descend from a branch of the Anglo-Norman family of de Nangle in Connacht which adopted the Irish name Mac Oisdealbhaigh. This is the first example of a Norman family assuming a Mac name. The use of the prefix O is erroneous, though it does occasionally occur in 17th century records. Costello families are now widely dispersed but the heaviest representation of the name is still in Connacht. When the sparse Irish population began to increase it became necessary to broaden the base of personal identification by moving from single names to a more definite nomenclature. The prefix MAC was given to the father's christian name, or O to that of a grandfather or even earlier ancestor. At first the coat of arms was a practical matter which served a function on the battlefield and in tournaments. With his helmet covering his face and armour encasing the knight from head to foot, the only means of identification for his followers, was the insignia painted on his shield and embroidered on his surcoat, the draped and flowing garment worn over the armour. The origin of badges and emblems, are traced to the earliest times, although, Heraldry, in fact, cannot be traced later than the 12th century, or at furthest the 11th century. At first armorial bearings were probably like surnames and assumed by each warrior at his free will and pleasure, his object being to distinguish himself from others. It has long been a matter of doubt when bearing Coats of Arms first became hereditary. It is known that in the reign of Henry V (1413-1422), a proclamation was issued, prohibiting the use of heraldic ensigns to all who could not show an original and valid right, except those 'who had borne arms at Agincourt'. The College of Arms (founded in 1483) is the Royal corporation of heralds who record proved pedigrees and grant armorial bearings.
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