I think it simply says 'British', indicating his nationality. Note that in 1851, Ireland was still part of the United Kingdom.
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:
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."