You are correct Michael, the Perceval Maxwells were, and perhaps still
are (not having means to check it at hand) a Landed Gentry family, and
of "unbelievable complexity" in terms of family history, according to
the introduction to the family papers at the Public Record Office of
Northern Ireland.
They can most simply be divided into two branches, the northern branch
formerly residing at Finnebrogue House near Downpatrick and at
Groomsport House in County Down, and the southern branch at Moore
Hill, near Tallow, County Waterford where, the family papers state, a
Mr John Perceval-Maxwell lives and farms.
It seems the family at one point ended up owning Amherst Island in
Ontario, Canada for some years, in addition to other estates in
Ireland, more details from the link:
http://www.proni.gov.uk/introduction__perceval-maxwell_papers.pdf