Iritis is an inflammation of the iris, the colored part of the eye, and this tiny poem seems to speak from inside that discomfort. The speaker is stuck in a long, sore stretch of time: "day and my eye / ill still." The words barely move. "Ill" and "still" nearly rhyme and nearly repeat, as if the sickness and the waiting have become the same thing.
The second stanza sharpens the feeling. "Pain plain as / an ember in amber" works through sound, with the short, tight echoes of "pain" and "plain," then "ember" and "amber." An ember suggests a small, glowing heat, which fits a burning, aching eye. Amber suggests something trapped and preserved, held in place. Together they give an image of pain that is bright, clear, and impossible to ignore, yet also frozen.
The mood is quiet, tense, and patient. The speaker doesn't complain at length. The poem is as short and confined as the suffering it describes.