You still haven't answered all the questions so the advice that can be given is minimal.
However, IF the title was not in order, and IF it was only sorted out recently, then the date that the title was sorted out should be the valuation date, not the date of death or the date of probate and a case should be made to revenue about that. If that were done then there should be no penalties.
Furthermore she could apply to pay CAT by installment. This brings interest but if she cannot pay otherwise, then she may find it an option to explore.
Finally, it sounds like proper tax advice should be taken.