Are you sure that a lender would give you a mortgage for a new house knowing that you have €100k of negative equity.
Which lender is your current mortgage with?
All of the active lenders allow borrowers to transfer their mortgage shortfall to a new property.
If you have a cheap tracker, you should probably consider staying where you are as long as possible as the interest rate is so low, you are paying off the capital very quickly. This would allow you to build up your savings even further.
That would not stop you from buying a site, but don't buy a site unless you know that you can get the money to build the house.
Brendan