While looking for something else, I found a piece of paper titled ABBREVIATIONS USED IN GRIFFITH'S VALUATION.

It appears to be a handout from an IGSI class and gives credit to Margaret Dickson Falley's book, Scotch and Scotch-Irish Ancestral Research, pages 89-90. I'm guessing this may be the work of long-time IGSI researcher/volunteer/librarian Beth L. Mullinax:

LAND

A = ACRES****

R = ROODS

P = PERCH    

VALUATION RATES

£ = POUND STERLING

S = SHILLING

D = PENCE

**** Acres in 19th century Ireland (when Griffith's was taken) are equal to today's statute acres. A statute acre contains 4840 square yards.

Prior to 19th century, an Irish acre contained 7840 square yards, being equal to 1.62 statute acres of today.

My note: A perch is one square rod. A rood is 40 perches.