It is impossible for any of us to know if the move back to Ireland will be a success for your father. 45 years is a long time and Ireland has changed a lot.
He may have lots of friends and family here that he has kept in contact but maybe he doesn't. Do you live in Ireland or will he have anyone close to him to
help him with moving, settling in, opening a bank account, and all the other practicalities of living in a new country?
Many people who dream of retiring in ireland are dissappointed by the vast difference between their memories of the country they left, their perception of what Ireland "should" be like and the reality today.
Having said all that, if he is realistic and wants to try, who am I to stop him.
If he chooses to move, I think it would be a mistake to buy a house, at least until he has lived here for a year or two and is definite about staying for the long term.
I am guessing that your sister wants to remortage her house to prevent your father from selling his in Canada. Is that correct? My advice to you, your sister and your father is to sell nothing, remortgage nothing until his long term plans are clear,
If he moves here, he wont be in a position to buy quickly anyway, as he will have to decide where to move to and look for a suitable house, and go through the slow legal process of buying. He will need to rent somewhere to live while he does all this. He should rent for a year or two and decide then what to do.