In OM10800 (the manual the LPC82x) under 12.3.2 there is a short description of the use of the DMA output triggers.
This is clear but does not bring mush info about how to set up the triggered channel in terms of settings of the channel configuration register. One can assume that the output trigger is a pulse, firing when the channel descripter is exhausted. One can then also assume that the channel must be programmed for HW trigger in edge mode, and probably rising edge. But this is guessing. I prefer a more formal description