Let's play along for one more minute...
If the train departed 0638 Roscrea then arrival in Ballybrophy would be 0701/0702. The 0654 would therefore have just left. Surely that stop would be deleted if this "interesting" plan got a go-ahead, which would accelerate the 0505 ex Cork by a couple of minutes or perhaps serve a more lucrative stop like Kildare instead?
Secondly, should the aforementioned train be a 2 x 22000 in the post 2700 era which seems to be approaching (and presumably the only reason IE didn't discount the idea by saying the 2700s couldn't keep pace on the mainline), could that second set then not turn back at Roscrea and form the Limerick commuter service instead? I realise there would still be the current northbound service to consider.