Whilst Banjo Ray makes the very important point that the GDAE banjo tuning is an octave lower than the fiddle (also the mandolin), the original tenor banjo tuning CGDA is actually an octave higher than the viola.
It`s all a bit complicated really, but the main reason for using the GDAE tuning on the banjo an octave lower than the fiddle is that one can play along on all the Irish tunes at a session with fiddles, flutes, whistles, etc. without any hassle or having to use the capo.
However, I feel that using the original tenor banjo tuning, CGDA, the banjo tends to sound much better, has a better ring and a more lively tone.