No sign of Sarderra in the 1851 list of towns and townlands
http://www.thecore.com/seanruad. It looks clear enough on the page, it might be noted in later records under some variation.
Some of the townlands and boundaries were not really formalized until the surveys were carried out for the 1st Edition OSI maps in the 1830s. Sarderra was possibly included under in another townland, or even, as I've seen sometimes, ended up in a neighbouring civil parish.
Achonry is quite a a large civil parish, consisting of about 80 townlands, plus two towns, but might be worth browsing the area on the historic OSI maps so see if there's any similar looking names :
Tobercurry town, Achonry civil parish (townland borders are indicated in red, civil parish borders in green/yellow)
S.
