You are clearly like a rabbit in the headlights at the moment.
Firstly you need to calm down and then you need to think logically.
I have had a few off-the-wall claims for tax from the revenue over the years and its pretty frightening. But neither time did I use an accountant and both times I ended up with no liability. But I was paying tax honourably (if that's the right word) and I had good records. One demand was due to a clerical error on revenue's behalf.(double counting income from a new job) and the second demand was them flying a kite.(off-shore accounts)
First ring revenue and ask them what the procedure is...explain that your record keeping has been a bit patchy. They will tell you what to expect and you can get cracking on filling in the blanks.
So you need to sit down yourself and estimate your income over the last few years.
Estimate the various costs and figure out what the likely income was.
Does this income match..your comings and goings over the years in your bank account, car,house,property, clothes, holidays?
If so what credits were you entitled to in those years (might need tax accountant for this part) but it will cost less if you have done the donkey work.
So what was your liability....add the interest penality and work out what you can pay...payment plan if necessary. Then approach the revenue.
Good luck.