I'm looking to replace some interior doors. However they measure 29.75" and standard doors are 30". If I was to by new, unused, doors privately is there anywhere, in the Dublin area, that I could get that 1/4" planed off at a reasonable cost?
You'll need to be very careful on your choice of door for that approach to work. You'll need to ensure the ones chosen are solid wood, and not engineered/veneered.
Rather than plane them, a guider router will likely give a better result. Whoever does it, ensure they take an equal amount off both sides of the door and not take the lazy approach of removing the full with from one side, which may throw off the finished look.
Woodworkers in Terenure have a full workshop, so should be able to carry out the work and advise on what doors they stock that will be suitable. If buying privately as you mention, any kitchen fitter or joiner should be able to do that for you, most of the bigger businesses will only deal with products they supply, there are risks working with products you're not familiar with that they won't be prepared to take on.