Padraig Mac Giolla-Domhnaigh

Halfpenny—This name in South Louth, in the district of Navan, Co. Meath, and adjoining parts, is the anglicised form of Halpin (O'h-Ailpéin).

I have also seen it stated that the name was originally of Scottish extraction in Louth, which would render it to be written Mac Ailpein, but of this I haven't seen sufficient evidence, though several Scottish Septs settled in Co. Louth at the beginning of the 17th century.

Alphabetical Index of Anglicised Surnames in Ireland

See also Woulfe’s Irish Names and Surnames
and O'Hart's Irish Pedigrees