Hi, Sarah, you don't say what you found out about "Inland Brith" or "Inih Land". Are these a reference to Inishbeg Island, by any chance? That would be quite a way from Drishanebeg (nearly 4 miles). And even further from Castlehaven Parish.
The Irish Times has a fascinating section of Top 10 surnames in each parish for 1851-1853. Limiting the area to West Cork, Murphy comes in the top 10 in a whole swathe of the northern part - and at No.1 in 11 parishes!!!
Curiously, Murphy does not figure at all in the Top 10 anywhere along the south coast WITH THE EXCEPTION OF CASTLEHAVEN where it comes in at No.6.
Wouldn't it be lovely to discover just what brought them down there?