I'd seriously consider your friend's advice - esp if the property is nearby. Assuming its a 2 bed, move back in (change bills etc. to your name) so it's your legit PPR. Now that you're renting a room in your PPR, your rent income can be legitimately tax free (up to the 14k limit:
https://www.citizensinformation.ie/en/housing/owning-a-home/home-owners/renting-a-room-in-your-home/ ), no need to register with RTB, tenant will be a "licensee", you've more control, you stay on the property ladder and you may even be able to shop around for a better mortgage deal/refinance...
(and if the rent now is 1400 - and assuming you pay tax etc. on that at about 50%, then currently you clear about 700. You say it may be worth 1700, so perhaps you'll find someone happy to share for half that or more (say 850 - esp if you spend a lot of time at your mother's) - and given that that income is now tax free, you may even come out ahead.. By living in it you're also increasing the proportion of time it will have been a PPR vs rented - which can have a favourable effect on any Capital Gains you may owe (though by the sounds of it, you wouldn't owe any at this point...)
The only thing to consider is if you were to inherit a home you've not been living in, you may not be able to avail of certain CAT exemptions should the value of that inheritance be high - there's probably advice elsewhere on this site about that - or see citizens info:
"Dwelling house exemption
If you get a gift or inheritance of a house that has been your main residence, it may be exempt from tax if you do not own or have an interest in any other house. There are conditions on how long you must be resident in the house before and after receiving the benefit."
Some types of gifts and inheritances are exempt from Capital Acquisitions Tax (CAT). Others qualify for reliefs that reduce the amount of CAT you have to pay.
www.citizensinformation.ie
Revenue:
https://www.revenue.ie/en/gains-gifts-and-inheritance/cat-exemptions/dwelling-house/index.aspx
Good luck w your decision!