The simple cross is a late motif when applied to Christianity. Crosses started off as Chi-Rho motifs and simply became simplified into what we know as a cross. The Christian symbol of the cross has nothing to do with the crucifixion, but originates in the Chi-Rho design.

Chi & Rho = the first two letters of Christ's name in Greek - Christos

http://www2.cumberlandcollege.edu/acad/rel/webpage/cross/crxr.htm

After looking like this the loop on the top of the Rho became a hook and then eventually disappeared altogether. Also the vertical shaft lengthened and the two diagonals moved closer together and became the horizontal bar.

http://www.ancientstoneart.co.uk/gallery/scot/s8echoid.htm

If you were to undertake a study of the early Christian cross slabs around Ireland you can very accurately date them by the design of the cross they carry.