There are a number of dates in an inheritance for tax purpose. There is the date of inheritance and the valuation date with each having a different meaning and function. The date of inheritance is, in most instances, the date of date. The valuation date can be the date when the grant of probate was extracted but there are exceptions. The key thing about the valuation date is that it determines when the CAT is payable and the IT38 tax form is due to be filed. If the valuation date is in the period 1 September 2017 to 31st August 2018 then the filing and payment is 31st October 2018 (with ROS extension little later).
The key thing about valuation date is that you need to be capable of having the assets transferred into your own name. If the share certificates are missing then the valuation date may not have passed as the assets may not be capable of being put into your own name. It is possible for an inheritance to have different valuation dates - one for cash and one for shares. Take a look at the this on the Revenue website:
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It may be worth your while talking to a CAT specialist.