When I was there in recent weeks a unit swap took place which meant the unit I had travelled on off the South Tipp line ended up going through to Colbert.
But I think the swap was only decided after arrival at the junction so everyone had already detrained.
There seems to be a spare 2700 set at Limerick Junction some (all?) of the time.
The train at platform 2 (end of bay platform nearest station) is essentially "trapped" whilst platform 3 is occupied.