Sorcha, I agree about harp construction & craziness in harpers/luthiers.
I have a Celtic lever harp - had some close calls - heat, humidity stretches the strings, requiring tuning up, and a cool change can snap strings or pop the soundboard if you don't tune down again.
Reconstruction is probably possible - but try not to sacrifice sound quality by putting too much badly designed reinforcements in the sound box. Try to use bracing rathing than gluing great big bits of wood inside and taking up all that precious empty air in there (sound vibrations need the empty space inside).
I love O'Carolan's music too. Check out Lesley Nelson's Contemplator site for lots of lovely O'Carolan midis.