The owner of a residential property subject to a non recourse mortgage essentially has a put option against the market value of the property. If the market price of the property falls sufficiently, the owner can surrender the property to the mortgage issuer and in exchange receive full offset of the cash flow liability of the mortgage loan. A similar but diluted put optionality holds for recourse mortgages if there are legal or practical limits to the mortgage issuer's recourse claim against the owner's future income. Previous research based on American data finds that put optionality is an important, but not exclusive, determinant of mortgage default. This paper utilizes a database of troubled Irish mortgages to analyze the influence of put optionality on Irish property owners' default behaviour. We find that put optionality is a very important explanatory variable for observed Irish mortgage defaults, complementing and strengthening existing empirical findings from US mortgage markets