AFAIK it makes no difference if you are married or not in relation to trs - it is based on whether it is single/joint mortgage and the status of each applicant, i.e. FTB or non FTB, when calculating the relief you are entitled to.
If you are both on the mortgage then both of you will be entitled to relief based on either status above.
I don't know if it TRS can be backdated though.
If you are still unclear after postings on this thread call the trs helpline - 1890 46 36 26. They are very helpful.