Two points:
1. Regardless of whether or not there is inheritance tax, a return must be made to revenue if you are going to take out probate, and this involves declaring a property value. In such circumstances, Revenue will not usually ask to see a professional valuation, so if the widow wants to simply value the asset herself, that would probably be adequate for probate (I make no comment on whether this would be wise - just that it is feasible)
2. The first question the widow should address is whether in fact she needs to take out probate. As regards the house, if she has no plans to sell or mortgage it, it can sit perfectly well in her husband's name until she dies, and the next generation can then take out probate\administration of the estate.