Many Celtic Crosses have the story of Man carved in relief in them, or it is the story of our Redemption, instead. The ones that are most recognizable, however, are the knotted crosses.
It was, originally, devoted to to a sun god, but the Christianization of Ireland changed the meaning of the Cross. However, the circle still symbolizes the Son/sun that rose in our darkness. The circle can also represent the endless love God has for His people.
Durrow High Cross
This magnificent 8th century Celtic cross from County Offaly, Ireland, depicts Christ in Majesty flanked by trumpeting and adoring angels and the Lamb of God, with additional Biblical scenes of Daniel and the Lion, David the Harpist, Abraham and Isaac and a scene which seems to depict the Trinity with the Word of God. In addition, there are four Celtic knots and four spiralling [sic] bosses, perhaps symbolizing the four Gospels.
St. Patrick and Columba Cross
This impressive 9th c. Celtic cross from Kells Monastery, Cty. Meath, is dedicated to the patron saints of Ireland, and depicts the crucified and risen Christ, surrounded by the four Evangelists. It is the only surviving cross to feature all four Evangelistic symbols: the man for Matthew (the humanity of Christ), the lion for Mark (Christ’s resurrection), the ox for Luke (Christ’s sacrifice), and the eagle for John (Christ’s divinity).
Clonmacnois Cross
The famous 9th century Celtic cross of King Flann, High King of Ireland, depicts St. Columba between two angels, the Last Judgement, Christ with Peter and Paul and the founding of the monastery with St. Ciaran and King Dermot.
Celtic Cross Wiki