Nitpick: We're not really talking about inheritance tax, are we, since the couple won't be dead? It's gift tax.
As Towger says, don't assume that it would be easy for an elderly couple to relocate to Australia for a period of years, purely on the basis that they have adult children living there. I'm not saying it's impossible, but Australia does not encourage this, or make it easy.
But, for the scheme to work, they don't have to move to Australia; they just have to leave Ireland for long enough to cease to be ordinarily resident in Ireland. No doubt there are cheap and sunny places that they could go to.
If they're not resident or ordinarily resident in Ireland, and the children to whom they are giving property are not resident or ordinarily resident in Ireland, and the property given is not situated in Ireland, then there will be no charge to Irish CAT on the gift. As Brendan points out, no need to incur the frictional costs of buying and selling a house to make this work.
But emigrating for five years may be a lot to ask of an elderly couple just so their children can save a few shillings in gift tax. If they've always wanted to live abroad for a while and see retirement as an opportunity to do that, great. But if they wouldn't do this other than for tax reasons, then such a radical change in life and lifestyle, disruption of social network, change of medical and other service providers, etc, is a lot of stress to take on just so that somebody else can save a bit of money. Just sayin'.