I think the issue is that the cable will overheat at the point where it connects with the pantograph if the train is stationary and drawing a lot of current. If the train is in motion, the connection point is in effectively in motion and so the resistive heating is only happening in the one spot for a short amount of time.
This battery idea strikes me as being a bit of a greenwash. At face value, the original idea of bi-mode makes a lot more sense if it's a contingency for delays in electrification. If the electrification is a couple of years late, they'll be ripping out a relatively cheap engine with a year or two to go before overhaul rather than a vastly expensive battery with a decade or more life left in it.
Still, at least the 50km range removes the temptation to run high-density commuter units to Sligo.