I think they can only write it down to fair market value on the balance sheet. Any statement otherwise from O'Leary is likely to be bluster.
Depreciation and profit on assets will end up having impacts on the level of tax paid and putting anything other than fair market value on the balance sheet could be perceived as an attempt to avoid paying tax in a given year.
Don't confuse reported profit with taxable profit. The 2 are completely different and its unlikely Ryanair pay any tax on profits because they also get allowances for investing in aircraft much the same as a manufacturing company get for investing in a new factory..........its why the Tax accountant who explained some of it to me gets highly paid.