As the first house was purchased in Holland, and if rented out (assumption on my part) is an investment you can't claim tax relief on the Dutch mortgage.
If the property in Holland is rented out you also have to pay tax on the rental income less allowable expenses in Holland and the same in Ireland (mortgage interest is an allowable expense in Ireland) not sure about Holland. You can offset the Dutch liability against the Irish liability when paying the Irish Revenue.
If you decide to sell the Dutch property you may be liable to CGT in both countries but you can offset the Dutch liability against the Irish liability provided the double taxation agreement covers this type of tax.