The phrase "most ironhearted" implies someone who is very cold, cruel, and unfeeling. Antonyms for this phrase could include "most tenderhearted," "most compassionate," and "most empathetic." These words describe someone who is kind, caring, and understanding, instead of being callous and unemotional. Other antonyms might include "most forgiving," "most gentle," or "most warm-hearted." These words evoke an opposite feeling to the idea of hardness or coldness. By using antonyms to describe someone, you can paint a vivid picture of their personality and character, and convey a different set of emotions to the reader.