What is ironic about Paul's mother finally loving him?

Although Paul's mother devotes more attention to him during his sickness, her solicitude is superficial at best. For reasons that the author has not revealed, Paul's mother remains emotionally detached from her children.
People who are unable to love often compensate for their emotional numbness by forming what is called a pseudo, or fantasy, bond with their loved ones. Emotionally wounded individuals resort to this type of defense mechanism to conform to society's ideals about love and loyalty.
As the text tells us, people like Hester show great gentleness or solicitude towards loved ones because it is the only way they can convince themselves (and others) that they are normal. So, when Paul becomes ill due to the immense stress he has been under, Hester manifests the expected behavior of a concerned, loving mother. This is the only way she can avoid undue scrutiny of her true self.
This pseudo-mother-love is the only kind of "love" that Paul will ever receive from his mother. Even on his deathbed, Paul must endure his mother's lack of real feeling for him. The text tells us that Paul's bet on Malabar (the horse) made his family over eighty thousand pounds. Yet, his mother remains impervious to everything he has tried to do to win her love. 
Her solicitude for him on his deathbed is even more ironic in the sense that Paul can no longer appreciate it. He has become emotionally numb in the process of trying to draw out his mother's love. His last words are of Malabar the horse and his propensity for being "lucky." Paul is now, by his mother's definition, "lucky" but still very much unloved.
https://www.psychologytoday.com/us/blog/the-human-experience/200812/the-fantasy-bond-substitute-loving-relationship

Actually, Paul's mother never is able to love him, even when he is on his deathbed. She does rush "to gather him up" in "tormented motherhood" when he falls from his rocking horse. She sits with him as he suffers from a brain fever before he dies, but we are told she is a "heart-frozen mother." The term "heart-frozen" means that, try as she might, she cannot love—she is incapable of it. Something has cut her off from her emotions. To reinforce this idea, Lawrence refers to her as sitting by Paul "stonily" and feeling as if her heart had actually turned to stone. She stays with her son out of a sense of duty, not because she loves him. The irony of the story is that Paul has given everything he has to try to earn her love, even his life, but it is simply impossible. He dies before he is old enough to know that she cannot love anyone.