We don’t know what pitch standards were used by the old Gaelic harpers.
This page explores some possible evidence and makes very speculative suggestions for the kinds of setups that may have been used at different times.
The most important things to consider when talking about pitch standards for early Gaelic harp is the pitch of na comhluighe, the unison strings that divide treble and bass, and are the system note or base note of the diatonic scale of the harp. Conventionally, these strings are given the note-name G, so the scale runs g - a - b - c - d - e - f or f♯. Alternatively, it is suggested that the na comhluighe strings can be named C, giving a scale c - d - e - f - g - a - b♭ or b. It's my suggestion that the alternative note names of G or C may have originally been used to distinguish between a scale with a sharp seventh (na comhluighe called C, 7th called b) and a scale with a flat 7th (na comhluighe called G, 7th called f).
I presume that the medieval Gaelic harp tuning and pitching schemes were related to the gamut of Guido of Arezzo. First written down c.1025, this scheme for vocal church music defines note names and relationships, though it is free-floating with no specific pitch standard in mind. Guido's system includes both B and B♭, which perhaps connects it to a harp system with na comhluighe at C.
Here it is set down to correspond with modern pitch standard (a=440). Middle c is indicated by c'. Frequencies are approximate and will vary slightly depending on intonation.
It seems to me that the basis of tuning a medieval harp should include this range. There is some discussion about whether a medieval harp might be tuned with an 8 note octave1, but I would prefer to see a 7 note octave with the 7th of the scale being retuned from b♭ to b♮ as required.
A Gaelic harp of 21 strings tuned to this scale would include na comhluighe at middle c', strings 10 and 11 from the treble or 11 and 12 from the bass.
In medieval times, pitch standards were not fixed as in later times, so although I have written modern A440 frequencies in the table above, in practice the harp would be pitched at any convenient level. The important thing would be to have the note names and the position of na comhluighe following this system at the chosen pitch level.
Some, if not most, medieval harps were bigger than 21 strings. For a 29 string harp, the additional strings could be added above or below the standard gamut listed above. My personal preference is to have an extended high range, which function more as sympathetic strings. Also, a harp of 29 strings could be retuned at will, by adjusting certain strings by a semitone, so as to set the standard gamut higher or lower on the instrument's range, without changing the overall pitch of the instrument.
Here’s a new suggested tuning for a 29 string medieval harp, using the lowest 20 strings for the medieval gamut (missing the lowest B of the gamut). This is at modern pitch, A440, and works well on a replica Trinity or Queen Mary harp with brass trebles and silver basses. It gives you na comhluighe on middle c, with two notes below cronan C.
A feature of Renaissance pitch standards is the strong evidence for the common use simultaneously of two pitch standards a 4th apart. This can most vividly be seen on the Ruckers 2-manual transposing harpsichords, where one of the two keyboards is offset a 4th higher than the other, yet both sound the same strings. It is also seen in "choir pitch" and "organ pitch", a 4th apart, and there is evidence for vocal music being set at two pitches, and of consort instruments being built at two complementary sizes2.
We can follow such thinking, and also respect James Talbot's offhand comment about the unison being at "about C (if not g)"3 by supposing that the same instrument, tuned to the same pitches, might have two different sets of letter names applied to it. We might also suggest that one would choose which set of names to choose based on whether the 7th from na comhluighe was tuned sharp or flat. Here is a suggested table. The pitch level given is one suggested as a possible level at which such a system may have worked in Renaissance music; we might call this A392/523.
For the 18th century Irish harp tradition, there are a number of ambiguous statements, but we can use Bunting's report on the Downhill harp4 to suggest that this may have represented a norm in tuning. Bunting does not tell us the pitch standard specifically, but he does say that na comhluighe G was tuned "to tenor G, fourth string on the violin"5 so if we could more specifically pinpoint the pitch standard of violins in late 18th century Belfast we might be able to refine this model. In the meantime here is a suggested table for an 18th century Irish harp, at a slightly higher pitch than modern.