If you're not good at writing essay style answers, definitely don't do history for the Leaving Cert as that is a huge requirement in the exam. I think leaving cert geography is similar to junior cert and just builds on the junior cert course. It's recommended to keep on a language and a science subject to keep third level options open. I suppose you should look at your junior cert results and figure out what your best subjects are. All of the Leaving cert subjects get harder and its best to do what you enjoy. Look at possible college courses that you are interested in and look at the entry requirements because you don't want to find yourself in sixth year, interested in a course but find that you can't do it because you didn't keep on a particular subject. All of th science subjects would be somewhat similar to their Junior cert counterparts but of course the content is going to become much more difficult. I'm not sure about this but I think the practical subjects may be similar aswell. The only practical subjects I did for the JC were art and home ec. In art at LC, there is a project but you also have to study art history. Home ec would also be similar, with a practical, a cookery exam and a lot of theory. I don't anything regarding the other practical subjects such as engineering, construction or dcg. I hope this helped you,, in some way. Good Luck picking subjects!
For practicals (excluding dcg I wouldnt really know about that) the theory changes a bit as far as I know but the theory doesnt tend to be difficult so if you have the hand skills and can design the project one or two of them can be handy to get you points