Martin's quote: "I cam across an authoritative architectural heritage reference which shows that both spellings (Ferns and Ferrans) are used in the area (County Meath)."

Yes, local townlands often have different spellings even up to this day, causing confusion for land registery, satnavs and ancestor research.

Here are some of the recorded spellings of Ferrans in the Placenames Database of Ireland.
http://www.logainm.ie/38986.aspx

Click 'data' and then 'text records'