Yes, it is ironic that the whispers of the house grow in intensity as the boy continues to rock on the horse.  That is the point of the story - chasing after more and more wealth doesn't bring true happiness, it only makes you want more and more and more.  And what better time than Christmas for the whispers to grow more audible?  Christmas has become a time of "unbridled avarice" (to quote a great movie, A Christmas Story), and I believe that was what D.H. Lawrence was trying to get across.

Is it "plausible irony"?  I suppose it's as plausible as the reader wants to allow it to be.  How plausible is it that a boy gets racehorse winners' names by rocking on a toy horse?  I think that if we can suspend disbelief to allow for that, then we can also allow for the irony of the whispers becoming louder the more wealthy the family becomes.

I think the links below will help you understand both the themes of the story as well as irony itself.  Also a re-read of the story will help it make more sense in terms of its themes and the sense of irony used by the author.  Good luck!

In "The Rocking-Horse Winner,"  the boy's attempt to stop the whispers only increases them. What does this irony say about the theme?

The ironic statement you make in your post is the beginning of a statement of the theme of this story.  This is the story of a boy and his mother who are never satisfied with what they have and always want more.  It is a very real and plausible situation for humans to have desires that just can't be satisfied but conversely feed the continual drive for more of whatever is desired.  In this case, the family never thinks they have enough money.  The real problem is that they live beyond their means, and even once the boy wins the money at the races and gifts it to his mother, it is quickly and rather frivolously spent. The bigger problem then is the need to maintain the most recent lifestyle expenses -- in this case, a very expensive education for the children.  There is something very sad and pathetic about these characters and any people who cannot ever be satisfied. And it is that very reality that literally drives young Paul to death.  He is absolutely consumed with the need to "see" the winner of the race, win the big money, and ease the imagined whispers for more money and therefore earn his mother's love and attention.