Harp Construction, sort of: Old Celtic harps were carved from a solid billet of wood, usually bog oak, and were not usually very large for portability. Nowdays, hardly anybody goes to that much work, but I have seen 1 that was made this way. I does reduce stress on joints. Most modern "folk" (opposed to pedal) harps are made in pieces. The Sound box/hollow body, the Neck at the top with the string pegs in it, and the Pillar, or front post. The sound box is usually made of at least 6 pieces to make a "box". The top/back of this rests against the players shoulder.
What often happens is that one or more or any of these joints is subjected to too much stress and the pillar or neck will start to separate from the box. Using the right kind of glue and adequete bracing on joints will help prevent this. There should be as little internal bracing inside the box as possible, or the harp will not resonate.
There is also a fine line between the necessary thickness of the wood to hold the bracing, and the necessary thinness of the wood to allow it to resonate.
Because of all this (and other!) harps are VERY sensitive to changes of temperature, humidity, etc.
It should not be too difficult to find a harp luthier in the DC/NewYork area, there are lots of Celtic musicians in the area, and you can always try Boston for a last resort. Good Luck!