Sounds like you are divorced or separated from the other parent and do not live in the family home, and pay maintenance for your children.
Did your current agreement run until the children were adults or until they finished full time education?
Your adult 20 year old son wishes to finish full time education and work part time with a view to returning to full time education after 12 months. Is he planning to move out? Have you, the 20 yr old and the parent sat down together to figure out how he plans to fund himself now he is no longer in education. Everyone needs to be clear on the costs, and where the money is supposed to come from. I would think for a 20 yr old that you would no longer pay maintenance to the other parent to cloth and feed them etc. He surely has an idea of how he is funding this gap year and if you are able to contribute you would give the 20 yr old the money directly?
I would see nothing wrong with proposing to the other parent that now the children are all adults that the family home be sold and you and the other parent fully separate your finances. You already have a housing solution for your children in full time education. If the last one is still in second level then it may be less disruptive to make the change until after the leaving certificate even if the child is over 18, as many of them are now-a-days. If the other parent wishes to retain the family home then they can use their share of agreed joint funds to buy you out if that is possible or otherwise help put it up for sale.
The first step is to look at you current agreement and see when it is supposed to end. Then communicate with the other parent and your children about the proposed changes and solutions.