Reducing UK inheritance tax liability

My father passed away almost 2 years ago. My Mum survives him and is also UK resident and fit and healthy. My sister is also in the UK and has 2 children, I live in Ireland, also with 2 children. We are considering how to limit IHT liability on the (eventual) passing of my mum. We estimate the estate to be currently worth around £1.25m, so significantly over the joint IHT allowance of £650k it also includes a couple of rental properties. A provision in my Dad's will allowed for half of my parent's house (up to his £325k IHT allowance) to be put into a discretionary trust (we have the choice over whether to do this). I think the benefit of this is that the house value is likely to increase more than the IHT allowance. We are also considering my mum taking out a whole of life assurance policy as the paid out amount would be outside of the estate.

Questions:
a) How will inheritance tax be applied given I'm Irish resident?
b) Does this course of action seem sensible? Any other suggestions?