Celtic crosses were a new and unique development, separated from paganism by centuries. Even in the early days of Christianity it was an international religion. New ideas, writings, symbols, art, etc. travelled with the flow of missionaries, bishops, and pilgrims to and from Rome, the Holy Land, and the outer reaches of known human settlement. Taking a step back to Roman Christian art, we see many examples of the Chi Rho (Christ Monogram) surrounded by a victory wreath (derived from Emperor Constantine's labarum).
"In the midst of the street thereof, and on both sides of the river, was the tree of life, bearing twelve fruits, yielding its fruits every month, and the leaves of the tree were for the healing of the nations." -Revelation 22:2
"To him, that overcometh, I will give to eat of the tree of life" - Revelation 2:7
"For unto this are you called: because Christ also suffered for us, leaving you an example that you should follow his steps. Who did no sin, neither was guile found in his mouth. Who, when he was reviled, did not revile: when he suffered, he threatened not: but delivered himself to him that judged him unjustly. Who his own self bore our sins in his body upon the tree: that we, being dead to sins, should live to justice: by whose stripes you were healed."
-1 Peter 2:21-24
"The crucifixion was, therefore, being interpreted by the Irish monks as the central event in the history of the cosmos, represented in symbolical form by the ring or circle of the cross." -Harbison, page 4.