Hello Thomas,
I guess you have a conventional manual handbrake, by the year of the car.
Assuming the pads and calipers are in a serviceable condition, then it maybe wise to totally slacken handbrake adjustment to a point where the cables can be disconnected at both wheels. You can then sharply apply the footbrake and ensure both rear wheels lock up properly and then fully release properly. It would be great if your friendly MOT station could retest the rear brake efficiency and balance to ensure they are working properly - oops, just realised Ireland - so Government testing??
Once you have established correctly working rear wheel braking , you can then tackle the handbrake cable efficiency. Do both cables pull and release properly? Renew if any doubt.
Your A4 may have the same set up as the similar year A3, whereby the cables pass through aluminium guide tubes, and they can stick within these tubes. New tubes will be necessary and I think these are main dealer parts only. Liberal application of WD40 may assist handbrake cable action and removal of tubes if necessary.
Kind regards,
Gareth.