In reverse order, for bike shop mechanic-ing, I don’t think you can do better than Norms Bikes in Belfast. There is also Mech Monkey in Belfast.
Northern Ireland seems to have even more antiquated access laws than other parts of the UK. There is a lot of off-piste riding, but the legality of those trails are sometimes called into question. One of the most popular riding spots is Tollymore – which is just a stunningly beautiful forest park. Rostrevor also has many off-piste trails. Those two places probably have the most elevation, but there is great riding to be had in many other places. Cavehill in Belfast is my local, which I has really excellent, and technically challenging trails. Helens tower in Bangor is also great – less elevation, but equally great.