I play Irish harp myself; mine is a 32-string harp built by an American company called Dusty Strings, which I taught myself to play because there were no other Irish harpers in my city. I didn't find it terribly difficult, because I was (and am) pretty devoted to the music, and I was pretty motivated to play. It is true that lever harp is simpler in technique than piano, violin or guitar, because it is tuned diatonically--the full chromatic range isn't available, and the harp is generally only in one key at a time. And, unlike violin, or uillean pipes, or clarinet, a harp sounds good immediately--you don't have to work up to producing a good sound from it.
Any musical instrument is fragile; as for heavy, a lever harp isn't as heavy as you might think, because the sound chamber is hollow. My harp weighs 23 pounds and stands about four feet tall at its highest point.
As long as the tuning pins are securely in place, a harp won't detune easily unless the temperature or humidity changes, like any instrument. You will get used to tuning each time you move the harp to another environment, such as from your house to your car to some other room; but an electronic tuner will significantly reduce the time you spend. They aren't expensive...be sure you get one!
Harp string sets are made for the individual harp, because of the great variations in size and materials for each maker. Contact the harpmaker you choose for pricing on string sets, or for individual strings.
As for nails--the standard teaching for lever harp is that you keep your nails short. Strings are played with the pad of your finger, the top corner of the fingertip; hand position is a bit like holding a glass of water, with the thumbs pointed up. Pinky fingers aren't used; they are too short and weak. That being said, after playing for 17 years, I recently decided that I want to play a wire-strung harp, which is played entirely with long nails, so I grew out the nails on my right hand, and am delighted with the sound; ornaments sound crisper and more precise, and I have just that little bit more reach. But as you are a beginning harper, I'd advise you to start with short nails for now.
I looked at the Hobgoblin website (I gather that you're in the U.K.); and the immediate red flag that I see is that the Glenluce harp is Pakistani-made rosewood. I'd advise avoiding that one; many Pakistani-made harps are of poor quality, cheaply made and cheaply priced, but not very durable, and they don't make a good sound. Many experienced harpers recoil in horror at the mention of the Pakistani rosewood harps. The Hobgoblin models look to be a bit nicer than many I've seen, but please be cautious--a poor instrument will never produce a good sound, and will limit your progress.
I'm a little more familiar with Stoney End harps, as they are made here in the U.S., although I've never personally played one, so I really can't vouch for them.
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