No, he cannot force a sale. But in the divorce joint property is there to be divided out. It would be best if they went via the mediation route, a lot less costly all around. But now they your Dad sent your Mum a solicitors letter it may already be gone past mediation.
Things to consider.
So when your Dad left there was 20 years left on mortgage and there were no dependant childrens
Now there is €20K left to pay.
On separation how much was left on the mortgage and how much was house worth. Did both parents contribute to the mortgage.
When ye were kids, up to age 23 if in full time education was your Mum a full time SAHM supporting the family and her husband through childcare and family maintenance.
Since the split has your Dad contributed to your mum. .
Has your Mum paid your dad anything, rent?
Are they both working with pensions. Your parents are in their early 60’s at their youngest, but are probably older.
Does your father, own or rent another house.
So best case is your Mum has worked, paid off mortgage herself and you Dad has very little equity in the home.
Say house at the time of separation had a €320K mortgage with €220K left to pay. If the house sold for€320K your Dad would get €50K as your mum had cleared the last €220K herself since separation.
There are so many scenarios but it is very doubtful your Dad would get 50% of the property value. So if your Mum had no where to live if the house was sold you and your siblings could gift your Mum the equity due your Dad and she could continue to live there. So there are lots of options.