It's not fishy. As I said, you are setting too much score by points. In a lot of cases- medicine, vet, pharm, law, dentistry and psych- points are 550-600 and justified by how difficult those courses are. But it's competition that drives them so high. Nursing is 400, teaching and midwifery are around 450, engineering and most business courses are around that or lower. But English and History is 500-510. The more competition, the higher the points. As a sidenote, medicine is not officially anything over 500 but the test added on to it means you need to have over 560 to stand a chance in gaining the total.
German is dragging that down slightly, it's not near as popular as French. Economics is an extremely popular TSM choice which brings it right back up again. If you combined German with Russian or Geography it would be below 400. That's how it works- just utter demand. Have no fear, it doesn't make the course any less impressive or difficult.
You cannot have an A2 counted that was done in a different year. All A2's you present for scoring purposes must be sat in the same academic year. This is because Irish students cannot mix and match- it's do or repeat all 7 subjects in one go or nothing.
It's much more unusual to be doing 4 A2's than 3. This makes it much harder to even compete for UK students. However once you are doing 4 it gives you a big safety net. But doing 4 is giving you a big safety net for UK unis too- you can fail one and they won't care, you can't do that here. You still need to do very well, just not be a genius. If you compare other courses to Durham etc. it's much much higher here. Medicine, Dentistry- A*A*A*A*, vet, pharm- A*A*A*A, Psychology, Law- A*A*AA. You won't find that even in Oxbridge. There's a reason why TCD is seen as extremely competitive- you just got lucky yours isn't one of the insanely high courses. You should be thankful, not suspicious. You're not comparing like with like- our exam system and grading is extremely different to yours.
No, you need to get more than the points to be guaranteed. Points can climb, because of demand. Trinity is well known for it. It's rare to see a course jump more than 15, 5-10 is the usual but that can screw lots of people over. If you get the exact points you will go through random selection- I don't mean last year's, I mean in the year you apply- it means you are tying for the last place and a computer picks the winner randomly. You do NOT want to risk that.
You don't need to tell people 'I went to a uni like Durham' etc. Trinity is very well known and renowned internationally. Inside the UK, obviously people know more about Durham but they will know that Trinity is also a great uni. Outside of the UK, everybody has heard of Trinity. It has a huge number of famous past graduates as well as its great rep- UC Berkeley is entirely named after one of Trinity's graduates!
You will also only pay €2000 a year fees for TCD. Living costs are higher and you cannot get loans here, you need to figure out whether taking the hit in the short term instead of the long term is a good idea for you as well.
You must have English, Maths and a foreign language to at least GCSE level. You cannot present BOTH english language and literature at A2. You cannot present media, general or ICT at all.