Triple antibiotic ointment can be applied to the eye - make sure it is just the regular Neosporin stuff and not the one with added pain relief. Give that every 4 hours around the clock for a week in case this is a corneal ulcer. Eye injuries are an emergency and you really need the vet.
Keep a fly mask on and give banamine for pain relief.
If this is uveitis and not an ulcer, you need a steroidal eye drop. But if you apply steroids to an ulcer you will lose the eye. Steroids are nothing to mess around with. Those are prescription drugs.
Eye injuries can go bad quickly. A friend of mine is a vet. Her horse lost one eye prior to her getting it. The other eye got a scratch, she took it to work with her, and despite the best care available, the eye got a fungal infection and she had to put the horse down.
Another friend had a horse scratch its eye. After a week at the vet, the horse still lost its eye. She had well over $5000 in vet bills.
I would get it looked at by a vet ASAP. If you are lucky it is a small corneal ulcer or conjunctivitis which should heal in about 7-10 days with the right treatment. Eye injuries are ALWAYS an emergency because what do you do with a blind horse? If the eye melts, you would need to put the horse down or surgically remove the eye. You can't leave a destroyed eye on the horse because besides being painful it will get infected.