I think it simply says 'British', indicating his nationality. Note that in 1851, Ireland was still part of the United Kingdom.
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**Added in response to the comments:** It certainly was not necessary for the nationality to be included, but it is not clear whether this was added by the enumerator or the householder when he filled in the household schedule. The instructions on the 1851 household schedule could be easily mistaken to mean that British Subject should be written after the birthplace of someone born in Ireland:
> WHERE BORN.
>
> Opposite the Names of those born in England, write the _County_ , and _Town_ or _Parish_.
>
> If born in Scotland, Ireland, the British Colonies, the East Indies, or in Foreign Parts, state the _Country_ ; in the last case, if a British Subject, add, " _British Subject_."