The circle which encloses the shaft and crosspiece is the most distinctive feature of the Celtic cross. A circle with no beginning and end stands for eternity. It also represents a victor's wreath or crown.

The descending dove is a symbol of the Holy Spirit. It is taken from the story of Jesus' baptism, where the Spirit descended on him in the form of a dove. [Matthew 3:16 ]
The three-rayed nimbus around its head identifies the Spirit as a member of the Trinity
A dove shown without the nimbus is a symbol of peace.

Alpha and Omega (The beginning and End) [Revelation 21:6]

Chi Rho (Χρ) The first two letters of the Greek word for Christ (Χριστός)
It is one of the most ancient of Christian Signs.

The four winged creatures on the stem of the cross, a man, a lion, a ox and an eagle are the four creatures in Revelation 4 surrounding the throne of God.
Also creatures in Exekiel 1 thought be drawing the chariot of God.
Since the earliest days of the Christian Church, they have represented to four writers of the Gospel.

The Winged Eagle - The symbol for John is the eagle, soaring into the high heavens, looking into the bright pure light of God. John delves into the deeper meaning of the person and work of Jesus Christ.

The Winged Ox - Because Luke portrays so winsomely the humble Christ, the symbol used for him is a lowly beast who bears our burdens, an animal also used in sacrifice.

The Winged Lion - Taking its imagery from its opening lines, "The voice of one shouting," or better, "roaring, in the wilderness, Mark's symbol is that of the lion, the king of all in the ancient tradition.

The Winged Man - The symbol for Matthew's account of the Gospel, depicting Matthew's portrayal of Jesus as being the seed of Abraham and David, and through his emphasis on Jesus' teaching, lays stress on our Lord's humanity.

Basking Ridge Presbyterian Church (brpc.org)
Celtic cross | CeltArts.com