If I wanted to create an ironic situation in a piece of writing, but make it "hidden", how can I put it in such a way that any reader will be able to see it and understand it?
Follow the way in which all composition is meant to sound human: read it out to yourself and attempt to conform it to what would sound ironic, even if it is the characters own inner-dialogue; there is no one way to seem ironic either, but it really depends what the definition of ironic is to the character as well.
Having said that, try to think of what definition of ironic is to the character or narrator, and go with that when writing the piece.
I find things are either ironic or they aren't. I think by it's nature it is both hidden and obvious. It's generally been used as an addition for the reader or viewer, highlighting the disparity between two things, whether it's in dialogue, situation, or action.
I believe if the writing and situation are clear, then it is hard to miss irony, no matter how subtle.
What kind of irony are you specifically after? What is the situation?