You can really choose any topic that interests you. 'Topical' topics are great (eg. climate change, emissions, health, exercise, viruses). They like to see jc students 'engaging' with current events, so I would recommend one of those. I calculated emmisions for my cba and got exceptional.
Try to show of your maths skills as much as possible. This means demonstrate your ability in every area of maths. For example, in my cba I created lots of equations to calculate things, demonstrating algebra. I gathered data from the CSO, created tables and graphs and interpreted it, linking into statistics. I also created a function using the site Desmos, demonstrating understanding of functions. You could include some geometry aswell, depending on your topic (they really love graphs so I would recommend including some statistics work).
Don't overcomplicate it, it will seem like you don't understand what you're doing. Make sure the maths is correct. Everything must be as neat as possible and presented well, maybe even include some images. I used a poly-pocket booklet to do this. I printed the written work and used graph paper for the maths and the graphs.
Hope this helps :)
To collect data I went onto the CSO website (cso.ie) and then onto databases. I used the tables I needed. When you get the data for your area you should put it into tables. I don't have my CBA, it's in school and they never gave it back anyway, but I'll see if I can find some of my typed up work.
I also used a document from a renewable energy company that did a survey in a different area to help me. It might look a bit confusing but if you take your time it will make sense. I'll see what I can find and post it here.
heya, I did redecorating my bedrrom. It was pretty straight forward. Other things you could do would be organize a holiday? One piece of advice would be have as much maths as possible in it becuase the more content the higher your marks