Between them these estates had planning permission for 180, 000 units, which roughly translates into a new residence for one in every 25 Irish people.

ECONOMIST: When the roof fell in

Although some of this planning is used for smaller estates, more standard planning (gifts within annual gift exclusion or for medical or tuition expenses, use of the lifetime exclusion of spouses, use of the marital deduction, etc.) is used to substantially reduce or eliminate tax liability.

FORBES: I'm Okay with Mitt Romney's Income and Estate Tax Planning

The bottom line is big estates require a lot of advance planning, which presumably the Kochs have engaged in, even as they pile up wealth within Koch Industries.

FORBES: Koch Bros. Could Pull A $40B Dividend -- But Probably Won't

Many wealthy people got pitches from their estate planning lawyers last year encouraging them to pare down their estates with lifetime gifts.

FORBES: Finally, Estate Planners Can Read The Tax Code -- Not The Tea Leaves