Hey there Op, I was in vet much the same boat as you, I left school at 13 and didn't go back until I was 18. I had very low self confidence and was very quiet.
I decided to go to a youthreach, now these places have a reputation of being very rough. Now was there people there that were rough, yes but there were also people like myself, it was a mixed bag and bullying was almost non existant as everyone who went there had a story, we were like one big family. I made lifelong friends and gained exceptional experience and self confidence. It truly was perfect for me. I did an LCA which was perfect to ease me back into education. You will get a training allowance for going to Youthreach which is the same as the dole.
Your other option is VTOS, now you do have to be 21 and on a social welfare for a period of time. It's a good program and I wouldn't worry too much about the ages, I would think the vast majority of people would be in their 20's.
I personally decided not to go back to secondary school as I did not want to be judged, plus I think by the time you get to 5th-6th year you will have kind of made your friends. But if you think you could do well why not?
Best of luck with everything, returning to education was the best thing I ever did, I am now in college working towards making something of myself and I couldn't be happier or prouder.