Hi Brokendown,
a-unfortunately you cannot claim tax relief on maintenance payments made for the benefit of children.
b- As far as I know the loan interest would be treated the same as a Top Up Loan so it would depend on whether you were already receiving the full mortgage Interest amount allowable for your circumstance i.e. first time buyer or other on your mortgage as it stands.
c- If you are caring for your child/ children over-night for at least one night a year and you are not co-habiting as husband and wife with anyone then you will be entitled to an additional one parent family tax credit which is equivalent to a married persons credit, you should contact the Tax Office for this and it can be back dated.
Depending on your personal circumstances you may be entitled to a Medical Card or GP only card if you look at the HSE website they have a calculator on it.
The Citizens Advice Bureau is great for advice on entitlements, you should give them a call.
Good Luck!