What appears ironic about the room with the woman, child, and fire place compare with Scrooge's accounting and his clerk?

Irony means the opposite of what we expect to happen in reality; therefore, there is nothing ironic about the room with the fireplace and the woman and her child. It is what you expect from a family, filial love, and the hearth was the center of the family, similar to perhaps the television today. So you see there is no irony in this image.

And you cannot really compare Scrooge's room and the room with the woman and the child, for they are very different.

Irony, I just do not see it in this room that is filled with what it should have, a family that is warmed by its love and the fireplace.  

Scrooge's room, however, is a totally different matter. There is much irony in his room, but that is a totally different question.