There are 'conventional' keys for most Traditional tunes played at sessions , and , of course , 'written' tunes should be played in the key they were writtne in - James Hill , for example , wrote a lot of fiddle tunes in Bflat (Flash Sod) .
However , there is nothing to stop any one playing any tune in any key , provided they do not expect EVERYONE to be comfortable with their choice at a session / tune swop !