The tune comes from an old Gaelic war song called 'Rosc Catha na Mumhan' written by Piaras Mac Gearailt in 1792. Loyalists have better flute bands but Republicans have better folk bands so their message gets across more hearing the lyrics rather than hearing a flute play the notes.
I don't think this tune in any way resembles 'come out ye black and tans'. I think you need to visit the wolftones site to get a correct reading on that.