Petrie, quoted in O’Curry (vol 3, 1873, p.294) says “the inscription is... the letters R.F.G. and, in Arabic numerals, the date 1672”. Armstrong (1904, p.70) presumably relies on this when he says the Kildare harp “was apparently made in 1672”.
However I am reading the numbers as “anno 1215”, and I do not know the significance of this in determining what year is meant.
The coat of arms also has been used to date the harp. Armstrong describes the arms as “the Fitzgerald arms, charged with a crescent in chief surmounted by a helmet and an ape for a crest’. The initials are presumed to be of Robert FitzGerald (1637-1699). Robert’s father was George FitzThomas FitzGerald, 16th Earl of Kildare; Robert’s son, also called Robert (born 1675), was 19th earl of Kildare.
The overall form of the Kildare harp and its high-headed shape would suggest to me a date around 1700, or at least after the mid 17th century. The black-and-white decoration and masks seem “Jacobean” and would fit with a late 17th century date.