A 121 on its own would struggle with eight packed Cravens and the RPSI's "red van".
I once saw a 141 trying to lift nine loaded Mk 2s on a Portadown - Belfast morning commuter local. Boy, did it make a noise! It was standing in for a failed NIR 111 which normally did this - at the time, it was the last 071-hauled train on the island. The 141 had been commandeered off the Dundalk goods, I think.
However, even if a 121 was well able to lift 18 bogies, never mind 8, there's the small matter of turning. there's no way under the sun they'd allow it to go nose forst, so pairing will, I guess, be mandatory.
The DCDR considered taking 124 some years ago, but opted for 146 instead, as the curves around the north junction area and elsewhere would have necessitated a 2-man crew; not always possible on a volunteer-run railway. If nose-first won't do on the DCDR, it certainly won't do on the Cork main line.... Plus, an 071 would be well able to haul a train of eight plus a potentially failed 134.
Mind you, if the 071 threw its toys out of the pram, the whole lot would be stuck.