First off, your parents will need independent legal advice for what on the face of it seems an improvident transaction for them.
Secondly in relation to the tax aspects, you are effectively getting a gift of €600,000 from your parents. There are two charges to tax here ( no CGT as it appears to be your parents PPR)- stamp duty and Gift tax ( or CAT). In relation to stamp duty, if you have owned a property before the rate will be 3.75% on €800,000 and if not, the rate will be 3% on €800,000- that is if it goes into your sole name- if it goes into the joint names of yourself and your husband you can double the rates above.
In relation to CAT- if the property were to go into your sole name, the CAT would be roughly €600,000 less €466,725 at 20% ( but you are entitled to take into account the legal costs, stamp duty, SGE etc). If it goes into the joint names of yourself and your husband the bill will be €300,000 less €23,336 at 20% roughly.
If you had been living in the house for a number of years, there would be a potential exemption from CAT. If your parents reserve a right of residence, there would be a further deduction from CAt, but it is unlikely any bank would approve a mortgage on that basis.