1) Yes. You can apply to up to 10 courses, which on the application you list in order of preference. These 10 courses can all be at the same university.
2)Yes it is true, if you are applying from the Uk mainland ie not Northern Ireland, you would need to send photocopies of your exam results to the CAO immediately after they are given out in August (The Northern Irish board is the only board to send them direct to the CAO). The CAO has a really useful handbook on their website which explains all this.
3) It is true for many competitive courses you need to take four full a-levels to have a chance of getting in and even with four A's at A2 you can still lack the necessary points. If you look on the Trinity website you will find a document entitled 'admission-requirements-2008.pdf' which lists the necessary points range to get in, however the way in which they say they award points to A-level applicants is overly optimistic and you would do better to look at a document called 'A-level & LC Equalisation Final.pdf' to see how they actually do it.