Does the wire harp have to be played with the fingernail?
No. In fact at the last Harp Festival in Belfast in 1792 of the 10 competitors the only one who played with nails was Denis Hempson, who was also the oldest at 94 years old at the time. They all played wire strung harps, but only one played with nails.
Derek Bell of the Chieftains played both gut and wire strung harp and played without nails.
Triplett used to make a wire strung harp designed superficially for those who played gut or nylon as well and so wanted to play with the finger tips instead of nails. The string spacing was farther apart than on most wire strung harps and the sound board was spruce instead of the harder woods with the intent of reducing the sustain on the strings so you wouldn’t need to do as much dampening of the strings.
I think the main problem with playing with the finger tips is that most wire strung harps have closer string spacing so it is hard to fit your fingers in. I have played around with wire and have no nails to speak of. It can be done although on most harps it requires a drastic alteration hand position. Also the sound is different without nails. If you really want that clear, bell like, ringing tone then you need to use nails. But it is a matter of preference.
Derek Bell sounded fine playing without nails after all, just different.