A mortgage top-up scheduled for repayment over the same term as a comparable unsecured personal loan would probably be the cheapest form of credit assuming any charges that might apply don't inflate the cost.
No - depends on many factors not least of all the amount of equity you own in your property for example. Note that it's generally advised that one use a mortgage top-up/loan consolidation as a once off opportunity for rectifying debts/finance problems and that you schedule consolidated loan top-ups over a similar term to the original unsecured loans rather than paying them over the full term of the original mortgage.
Only for loans over €25,000. Their other rates are not as competitive as other lenders. If you are borrowing that much, a mortgage top-up repaid over a short term makes more sense.