Before I give advice over the internet, let me just begin by saying that the internet isn't the best place for advice because of an absence of details. That said...... I have been through this whole process. It formed a significant part of my life for 18 months and we successfully got our estate finished.
You ask "are the coc required to foot the bill?" No, unfortunately not. And I know that seems unfair. The law as I understand it is as follows.
The council has a discretion in relation to the value of the bond, and they have a discretion in accepting an insurance note or not. They are only in the wrong if they have been negligent. But look at it from their side. When a builder starts a housing estate, the council don't know how much to ask for. What if a builder is building an estate with 20 houses? Do they ask for a bond of €10,000 in case he doesn't finish the foot paths? Do they ask for a bond of €20,000 in case he doesn't finish the foot paths and the street lights? Do they ask for a bond of €80,000 in case he doesn't finish the foot paths, the street lights and the roads? Do they ask for a bond of €500,000 in case he doesn't finish the foot paths, the street lights, the roads, the boundary walls, the supply of mains water and the entire sewage system? If they asked for that kind money upfront all the time they could potentially bring alot construction to a halt. So they pick a reasonable value for a bond and go with that. Most times the bond is enough. In your case it isn't. In my case it wasn't. We got an estimation of how much it would cost to finish the estate, learned how much the bond was and asked each houseowner to pay the rest. It wasn't fair! But it got results! 95% of people agreed to pay €2,000 each. That was years ago now, and with the benefit of hindsight they all think it was some of the best money they ever spent. But again, it wasn't fair, but it was the quickest easiest way of getting the estate finished.
The law society publishes material on conveyancing, for students studying to be solicitors. In one part of that material there is a reference by a judge stating in his opinion the council should pay to finish estates. But it wasn't something said in court, and that undermines the legal relevance of what he said. The law may be changed in this area in the future.