The O'Ffogarty harp is made from three large pieces of unidentified wood (often described as willow; said by Armstrong to be "black sallow"). Like the majority of the old Gaelic harps, these are the one-piece soundbox, carved from a single block; the neck or harmonic curve; and the fore-pillar. The back of the soundbox is closed by a separate back-board. The harp measures 3 feet 8 1/4 inches (112cm) over its longest dimension.
The soundbox is 3 feet 1 inch (94 cm) long, including the projection into which the lower end of the pillar fits. The lower end of the soundbox is formed into two curves so that the projection nestles between them. The soundbox has six soundholes on the front, four of which are decorated with incised circles. There are holes for 35 or 36 (accounts disagree) brass strings, which are protected with metal "string shoes".
The separate back has apparently at some point broken and been repaired or replaced with a collection of pieces. It has two large access holes in it.
The forepillar has, as do the majority of the old Gaelic harps, a pronounced T-section running up the outside face. At its widest it is 4 1/2 inches (11.5 cm) across. The four holes on the top right hand side mark the position of the gold plate attesting Cornelius O'Ffogerty's ownership of the instrument.
The neck is decorated with incised lines running parallel to its edges. It has a large and distictive spiral carved at the upper end. Each flank bears a brass strip, through which the tuning-pins pass. There are 36 holes but only 35 pins. The neck is 2 feet 0 3/4 inches (63cm) long.
The shortest string is 2 1/2 inches (6.5 cm), the longest 3 feet 0 1/4 inches (92cm) long. The brass strings now on the instrument date from when the instrument was restrung around 1980. The strings would originally have been of brass, with possibly some precious metal in the bass.