You have a home loan rate on a mortgage of €58k on your family home. Is your question what happens to the mortgage if you let out this house? While the contract allows BoI to switch this to a buy to let rate, in practice, they do not enforce it. But so what if they do? The rate will go from around 4% to 6%. So you will pay an extra €1,000 this year and probably about €3,000 in total over the remainder of the mortgage term.
You would prefer not to pay this, but it's completely immaterial in the context of the overall numbers you are talking about here. So don't let this impact your decision in any way.