It is possible that the Cross was made for Cong
Cong was an important church patronised by the O'Conors in the twelfth century. It was recognised as a cathedral church at the synod of Ráith Bressail in 1111. Turlough O'Conor founded the Augustinian Abbey shortly after a fire there in 1137. Turlough's son, Rory, the last high king of Ireland, died in Cong in 1198. Rory and a number of his sons and daughters lived in retirement in Cong.
Throughout the centuries, the Cross was in the care of the Augustinians. The historian Roderick O'Flaherty saw it in 1680 and he copied the inscription. George Petrie, the antiquarian, saw it in 1822.
The last Augustinian Abbot of Cong was Fr. Patrick Prendergast, who was also the parish priest. The Cross was among his possessions when he died in 1829. His successor, Fr. Michael Waldron, sold the Cross to mathematician James MacCullagh for 100 guineas. MacCullough gave it to the Royal Irish Academy in 1839.
Another parish priest of Cong, Fr. Patrick Lavelle, attempted to steal the Cross from the Academy to return it to Cong around 1870. He walked into the Academy and asked to see it. Lavelle tucked the Cross under his cassock and walked out with it. However, he was caught and arrested before he got to the railway station.
The Cross of Cong was transferred from the Royal Irish Academy to what is now the National Museum of Ireland in the 1880s.
Read more about the Cross of Cong