Use eye wash saline to wash out inflamed eyes or those with foreign material visible in the eye. The saline must be sprayed at the eye and this can be difficult to do with commercial dropper bottles. It is generally not possible to ask a horse to tip its head back while you drip saline in its eye.
Personally I prefer the use of a syringe with sterile saline for washing the eyes of horses. I find it easier to control the stream of saline. A 20-35 cc syringe is less bulky and more precise. A broken off 18 gauge needle creates a strong stream which can be easily controlled and aimed.
The advantage of a commercial product, however, is the packaging, and convenience of having a closed, clean dose when needed. Eye wash saline is provided in a variety of dropper and squirt bottles.
In an emergency, saline of approximately the right strength can be made using 1 cup of distilled water mixed with 1 teaspoon of table salt. This can be drawn up in a syringe and squirted in the eye.