Since no one has posted a specific brand of brush yet I will make my recommendation. I use to struggle a lot with eye lens but recently found a brush/technique that works extremely well.
The brush is a Tamiya Modeling Pointed Brush (Fine). I tried a few other "fine tipped" brushes but for me, this one works perfectly, the tip is unbelievably sharp and stays that way with a speck of paint on it. It's a bit pricey but I became much more satisfied with my lens work with this brush, my last posted models have some close ups of the lens if you want to look at them.