The concert will be at night, yes. The musicians do not play in total darkness, they usually have small LED lights on their stands, but they are insufficient to light anything more than their music scores.
A generator might be required depending on the barn. I think we can rent or borrow one from a local farmer, they usually have them.
A string of lights is an excellent idea. How low should the wattage be in order to be considered "low"? Would 25W be ok?
I guess that non-incandescent bulbs are less desirable because of the spectral composition? What about white LEDs?
Conventional incandescent: 15w bulbs would be fine. Maybe too much, if you want to keep the night feeling.
Another thought occurred to me: Why not use the strings of LED lights that are so cheap now. The kind used for Christmas decorations. Best if they are the "tungsten" or "warm white" types. They are so efficient, a sufficiently long string(s) of them could be powered by one automobile-sized battery for many hours, through a relatively inexpensive inverter(s). Dead silent, light weight, easy to rig(depending on the location).
Added idea: If rigging above is impossible, they could be laid on the floor(easy), or hung from stands in bunches.
Two light sources, one at either side, or one each at front and behind the players could be sufficient.
EDIT: If mains AC power is available, so much the better.