Very difficult to find without directions to a record, for example.
I've managed to find that the place name is spelled North Petherwyn in 1881 (and actually is spelled that way in the census book in 1871 as well). Pages 7-8 of district 3 in 1881 show the address "Poor House" but there are separate household schedules for each of the handful of families and the adults are shown with occupations. The previous address in 1881 is "Rough Ground".
The 1881 poorhouse households start with Charles Lawrence, farm labourer, with adult children who are general servant and farm labourer. There are several farms on the previous pages so it would seem the labourers had employment. Could it have been a temporary residence, for workers in the sprint planting, for instance? A majority of the heads of household were born in North Petherwyn ...
One of the households is Philip and Mary Parsons. In 1871 they are at "Ford" between Ford Farm and Ford Hill.
Finding the address in 1871 would mean reading through all the records, without more specific directions. Can you give some, e.g. to a particular household? I'm just thinking that identifying the location might help some, although not likely.
www.workhouses.org.uk just says the Launceston Poor Law Union covered North Petherwin:
http://www.workhouses.org.uk/Launceston/Ah. Looking at the description of district 3 in 1881:
All that remaining part of the
Parish of North Peterwyn lying to
the West of the two former divisions and
West of the road leading from Penheale
Ford to Penruse(?) to Winnacott, Billacott
Poor Houses and Trosswell(?) to Pattacott(?)
inculding the Villages of Penruse
Billacott, Poorhouses, Trosswell(?) Lower
Trosswell and Trossell(?) the Farms
Winnacott, Penruse Mill Hilford
Widdacott, Pattacott, Cappadocia
Rough Ground and the Cottages
of Billacott Hill Greenlane
Bickels House, Lake, East Trosswell
and the Trehouse(?), Griss(?) and Winna
Ford
and of district 3b in 1871:
... - including
the Villages ofPenruse, Billacott,
Poor Houses, Trosswell
and Trossell - ...
-- I'd say it was a place name, not a term for a "poorhouse".
In 1871 the households at that location start at page 10 of district 3b and there are several of the same famlies as in 1881: Lawrence, Cann, Bickel.
So it was a place of permanent habitation, with a fairly stable population.