What happens if I die before the 15 year limit is up?

I understand that I can gift €100,000 to each of my two children (no complications with multiple marriages or other children). 

After 15 years I can do the same again. If I die within 15 years does the €100,000 go back into the estate for inheritance tax purposes? 

Does the same apply to the €31,865 that I can gift to them each as a cash payment. Also is there a maximum amount in my assurance vie that shall be free of inheritance tax when I pass away?

Yes the gifts you mention will come off your children’s inheritance tax allowances if you were to die before the 15 years have elapsed.

The same applies to both the basic €100,000 threshold and to the additional amount for money gifts to close family.

Taxation of an assurance vie (AV) policy is separate and not included in the normal inheritance tax allowances and not treated as part of the estate.

The tax-free allowance for money left in an AV policy depends upon the age of the donor (and not the relationship to the recipient) when the policy is established: if the donor is aged under 70, then the tax-free amount is €152,500 per person – after which there is a set tax rate of 20%. The allowance is only €30,500 if the donor is older than 70 and then inheritance tax is applied but only to capital sums paid in, not interest (which is tax-free). See here for more about AV.