If you think he genuinely doesn't understand the position he's leaving you in, then talk to him and explain. If he fully understands, which sounds more likely then it's abuse.
You will need to gather as much of the financial information as you possibly can, and take it to a solicitor. Your local legal aid board may be able to represent you, or organise a certificate for another solicitor to do so. I think there is a payable contribution for this though.
Child maintenance is organised by the courts. The judge will assess your DC's needs and his income. Your local court can award up to 150 euro a week. If he should be paying more than that, you go to the circuit court, I think, but your solicitor can advise. Generally speaking, the mother is awarded the house, sometimes outright, sometimes until the youngest child turns 18 or leaves full time education. Ireland doesn't have tax credits, but you should be eligible for jobseekers, or lone parent payment. I think you can keep up your business, depending on the turnover, and if you're in receipt of certain benefits you could also be entitled to back to school clothing and footwear allowance, fuel allowance during winter, rent supplement, medical cards, and possibly the household benefits package to pay TV licence, and phone bill. You may also be entitled to free or subsidised childcare, but it tends to be a specific provider, which may or not be suitable for you. Depending on the age of your DC's, you could have a free year in preschool - ecce, which entitles your child to a full year for free, usually the year before school, but there's another scheme that offers a reduced rate. I'd imagine that you're already in receipt of child benefit.
I realise that it's so much easier for all of us to type on here and say ltb, while it's much harder for you to think about, let alone do. Unfortunately, having kids doesn't mean that the relationship would, or should work.
Newmama, IMO, if there's love and respect in a relationship, financial matters are shared equally, or according to the financial divide. In our house, all money is family money, but I know that this doesn't work for everyone.