Belfast hourly could easily be run with 5 sets (+1 spare) especially if the current idiotic time wasting schedules were tightened up. There is often sufficient time in Drogheda or Dundalk on northbound trips to nip out to the local shops while the train idles away at the platform.
Mk4s would be little addition to the Belfast line although cerifying them to run in NI would not be a big obstacle, they are not massively different operationally to a De-Dietrich but would require upgrades to on board signalling equipment, notably TPWS and the rather less useful DRA.
As for 9 piece ICRs in passenger service, this is a comparitiv rarity, perhaps less than half a dozen times a year. Eight piece ICRs in passenger service are also relatively rare but do appear during weekend engineering works on the Cork line when service frequency is reduced to two-hourly. No regular workings to my knowledge. Seven car sets are common enough particularly on commuter oriented services notably from Sligo, Newry and Maynooth, perhaps less so on the Heuston lines where 3 or 4 car sets appear to pre-dominate.