I have highlighted the problem of people splitting up in negative equity in this
Key Post
I suggest that you start by studying this in some detail.
I have already sought professional legal and financial advice however I seem to be going around in circles!
As a high earner who has a joint and several loan with someone who has defaulted, you are in a very bad position. A brilliant solicitor or a brilliant financial advisor can't rectify this.
Let's look on the positives in this. You have an apartment worth €225k.
This is costing you €8,700year in interest. To rent anything similar, would probably be higher.
So first decision: Pay absolutely nothing more than the interest only. If you pay capital, you are paying the debts of your ex. BoSI won't like this, but you have no other choice. This is risky as I point out below.
Second decision: Write to BoSI and set out the situation. Tell them that you are willing to sell the apartment in an orderly manner to maximise the money which they will get. But they must agree to write off all claims against you i.ie. they must write off the shortfall. Unfortunately, your ex will have to agree to the sale as well or they BOSI will need to go to the High Court to seek repossession.
BoSI want out of Ireland and may agree to a deal. They probably won't write off the full shortfall, but they will adjust it for the tracker - so maybe they will agree to write of around €80k.
As your ex is part owner of the house, he has to agree to the sale and may refuse. In which case, you either go to the High Court or BoSI get a repossession order.
You should agree to pay the €20,000 student loan off in full as part of the agreement. It has nothing to do with your ex.
There is a risk in switching to interest only, but it's a risk that you have to take. I presume that you have a high income as you refer to a partnership. Under the circumstances, BoSI may go after you for any shortfall if they don't accept the agreement with you.
The Personal Insolvency Bill could be more harm than good to you
It's aimed at people who are insolvent. If the cost of servicing your loan is only €4,000 a year, you will be able to meet your repayments as they fall due. You will not be insolvent. You could argue, that as your liabilities exceed your assets, you are insolvent, but that is not the definition of insolvency in the Bill.
Your ex could conceivably declare bankruptcy or go for a PIA under the bill, and could escape his liabilities. You would then be jointly and severally liable.
You could take advantage of this by going bankrupt in the UK, but I presume this would finish your career?
You may have to let your ex off the hook
It's possible that you may be able to do a Personal Insolvency Arrangement under the new bill. But BoSI could easily veto it, so you need their agreement.