The Gaelic harp tradition died out in the 19th century. Harps played in Scotland and Ireland since then have almost all been of a modern European design with only a superficial nod towards the old tradition.
Just like other instruments: lute, harpsichord, serpent, the early Gaelic harp has been revived by 'early music' enthusiasts, who build replicas of old instruments and teach themselves the technique and music by studying period books and manuscripts.
However, unlike most other instruments and musics being revived from historical study of written and material sources, Gaelic harp music was a high status oral tradition.
This has a number of very important consequences. The harpers did not normally write their music down. Therefore, what written sources we have are all second-hand accounts, most of them in English rather than the Gaelic/Irish used by the old harpers in Scotland and Ireland. Their music was memorised rather than read, and was composed in performance rather than on paper.
There is no need to despair. We don't need to give up altogether, or turn to fantasy and imagination. There is a huge amount of information locked away in libraries, museums and archives; there are many ancient instruments in fantastic states of preservation in museums. Much of this material has not been studied by academics. But by carefully piecing it together we can build up an amazingly complete picture of the old Gaelic music, and re-create the old tradition.