The terms of the rent-a-room scheme state that a qualifying residence within the State is occupied by the owner during the year of assessment. As you are not occupying the flat, it probably does not qualify for rent-a-room relief and so rental income from your girlfriend is taxable in the normal way.
With regard to your secondment, the key issue is whether it is voluntary or enforced. If the reason you are abroad for a while is because it is absolutely necessary e.g. to do a training course that you need to do to be employed, then there is scope for Revenue to regard your place in Ireland as your PPR.
However, if it is the case that you volunteered to go abroad for whatever reason, then you are treated no different to anyone else who goes abroad to work i.e. you become non-resident & your house in Ireland ceases to become your PPR for the period abroad. This may seem harsh, but in the end of the day, going abroad to work for a couple of years is a lifestyle choice on the part of the individual.
I asked about your girlfriends interest in the house because one common tax scam going around right now is, if a couple are buying a place and only 1 of them is a FTB, they sometimes only put the FTB's name on the mortgage and deeds in an attempt to evade the payment of non-FTB stamp duty. This is illegal as the non-FTBs beneficial interest in the house must be declared to Revenue regardless of who's name is on the paperwork.