All I can say is that there is a considerable amount of interest and tickets are selling well. It isn't my gig but I do know how much the show costs and with the capacity of the Picturedrome (which is smaller than the Irish Centre as I remember it) then I don't think it's unreasonable.
But Alio and John Martyn Fan have two choices! There will be other things happening which won't cost a penny so there's no financial excuse for not coming.