If you can find a way to make your composition work with limited DOF, then that's great, and this is often the key to good macro photography, but if what you have in mind really needs more depth, read on.

If you can get enough depth of field with a small aperture, then that's the way to go. If you are buying a macro lens, pay attention to how far you can close the aperture down.

If you don't quite have enough DOF, try backing off and using a smaller part of your picture area. You have enough megapixels to sacrifice some.

Another way to do it would be to use a camera with a smaller sensor and decent close focus. Given good enough lighting to use a low ISO, you'll likely get a better result out of a 5 year old canon Ixus for 50 bucks on ebay than you can get out of any macro lens on any SLR, because thanks to the tiny sensor, the DOF is outstanding.

There's some fancy tricks you can do by blending multiple images with different focal distances. These are mostly useful for stationary subjects and a tripod though. Not so useful for a live eye.