Here's my reasoning.
Ten year Euro bond rates are at 4%, I calculate an equated yield on Irish residential property at 3-4% in excess of the Ten Year Bond to reflect sector risk. The risk weighting is my take on factors such as
1. Falling real rental incomes
2. Substantial supply of empty properties
3. Market reliance on 'transient tenant' sector
4. Serviced land supply equivalent to 350,000 new units
5. Unsustainable rate of debt accumulation.
5. Irish economy's reliance on American MNC for export earnings.
6. The probable impact of American trade and current account deficits on the American economy and the dollar.
7. Rising Euro interest rates.
8. Heavy Irish exposure to risky overseas property bubbles.
9. Poor financial literacy amongst many market participants.
10. The deflationary impact globalisation will have on incomes growth in western economies in coming decades.
11. Mortgage fraud and unethical business practices in the lending and property sectors.
So my considered take on the extent of over valuation for Irish housing is in the region of 50%. Worth noting that prices have fallen in Tokyo by 70% over the past decade and a half, by 50% in many other bubble markets in the past.