What started as a power struggle between the South Sudanese president, Salva Kiir, and the former vice president, Rik Machar, turned into full-scale civil war, in December 2013.
The UN estimates that tens of thousands have died so far, and more than two million people are displaced within the country.
The organisation said people have seen their loved ones killed and their property looted and destroyed.
It warns that families face dire living conditions in makeshift camps, and are struggling to keep themselves and their children alive, as the country continues to unravel.
Cathy Hynds, CAFOD's South Sudan representative said: "I've lived in South Sudan, and know of the great promise the world's youngest country has to offer, but the continuing conflict has lead to bleak humanitarian conditions, which has exacerbated the food crisis in the country.
"People are likely to face dangerous levels of food shortages in the first months of 2018."
The charity said "lean season", when households run short of food before the next harvest, is forecast to start in January 2018, three months earlier than usual.
In Yirol in central South Sudan, CAFOD and its Irish sister agency Trocaire have distributed food and essential items to people most in need.
CAFOD's local partner, Bishop Barani Eduardo Hiiboro Kussala of the Catholic diocese of Tombura-Yambio, recently conducted a meeting of religious leaders and governors from nine counties of South Sudan to seek ways of "promoting greater peace and prosperity to the present and future generation".
He said: "In every part of this country, hatred has buried our youth in unknown graves.
"Violence is easy, but for the poor, all it means is agony and displacement.
"To achieve progress, we must take the harder path of breaking down barriers, building bridges and standing up for tolerance and diversity.
"Peace will benefit all.
"A country with such a young population cannot squander its wealth in wars that nobody wins."
