Direct shipping between ireland and europe is miniscule at present.
Between Ireland and the UK there are 4 ports serving ireland (Belfast, Dublin, Rosslare and Cork) The destinations are Cairnryan, Liverpool, Holyhead, Fishguard, Pembroke and occasionally Roscoff and Santender.
Apart from the last two destinations there asre multiple sailings each day to and from Ireland to the UK. An appreciable component of the "cargo" comprises HGV's. The infrastructure at Cork and Rosslare is probably inadequate to cope with additional traffic should other ports no longer be used. These changes need to be happening now unless seamless cross border movements are agreed. Prevarication and bullheadedness will impact Ireland far more heavily.