The time of day is ironic because it is dinner time and the end of work, but also the end of the boy’s life.
Irony refers to something unexpected happening. It means that there is a double meaning or an unusual occurrence.
At the beginning of the poem, Frost establishes the time of day right away.
And from there those that lifted eyes could count
Five mountain ranges one behind the other
Under the sunset far into Vermont.
Sunset, and supper, are symbolic of something ending. As the day is coming to a close, the boy’s life is coming to a close.
The speaker comments that he wishes they might have let the boy come in just a little early, because it would have changed everything. If the boy had come in before his sister distracted him by calling that it was suppertime, and causing him to lose his hand—leading to his death.