The required 'A' level entry grades can vary from year to year. For instance, entry in 2012 requires 'A' level grades of ABB. Applications are very welcome from those alternative qualifications, e.g. Irish Higher Leaving Certificates, BTEC, GNVQ and International Baccalaureate. Queries about alternative qualifications should be directed to the Admissions Office.
There is very little to choose between different ‘A’ level, or equivalent, subjects as a preparation for studying psychology itself. The fact that you have an ‘Arts’ background, for instance, is not going to be the handicap that some people think it could be. It is necessary to have a GCSE Grade C (or equivalent) in maths, but the mathematical skills required for the course are not substantial beyond using statistical tests for understanding the results of experiments and investigations.
The majority of entrants will not have studied psychology before - and this is absolutely no disadvantage. You might also ask whether you are likely to be interested in psychology if you have studied particular subjects and found them interesting.
If you have studied sciences and found they suited you, why consider psychology? You may well find psychology also suits you, since it involves the same concern for rigorous and ingenious experimental work. In many areas psychologists have attempted to find elegant laws and models comparable to those in the physical sciences but here applied to human and animal behaviour.
If you have studied languages, you may be drawn to the large area of psychology which is concerned with language, and communication.
If literature fascinates you, you may find it satisfying to study the motives, attitudes and thoughts of people from another angle, the psychological.
The same could be said for history, as much of psychology, like history, involves piecing together apparently isolated facts until a pattern can be seen in them.