Hi Mark
I presume that you are based in Ireland?
There is absolutely nothing below board about your family lending you money to buy a home or to replace a mortgage.
Is it a good investment for them? It depends on the rate you are paying on your mortgage. Presumably you will be paying around 5% when the fixed term ends. They can get around 4% on a deposit, so it's an ok investment for them, but not great.
They would have to pay DIRT on the deposit interest from a bank. They would have to pay tax at the marginal rate on interest paid by you.
You can claim TRS on the interest paid on your mortgage. I presume you can claim it on the interest paid to your family as well, although it would be messy.
If you are able to make your repayments to the mortgage company without any difficulty, I think that you are better staying with them and not complicating it unnecessarily.
Brendan