And Heysham-Belfast Stena freight service, which shares the Liverpool terminal in Belfast, though it has its own berth.
Seriously though, between two Berths in Holyhead (when the frequency can be maintained with one by retiming some ferries), and the many alternative routes*, I think there's probably plenty resilience provided all parties act reasonable (unlike P&O a few years ago).
*At least the below, I may have missed some:
-Rosslare-Fishguard/Pembroke/Cherbourg
-Cork-Roscoff
-Dublin-Cherbourg/Birkenhead
-Warrenpoint-Heysham
-Belfast-Birkenhead/Heysham/Cairnryan
-Larne-Cairnryan
-and technically you can go via the Isle of Man as well
The real reason Holyhead is so busy because there are lots of goods coming from the Northern/Midlands metropolitan areas that are stocked in Irish supermarkets and other shops.
The French routes aren't really useful for this since Brexit as they have to cross a customs border several times if routed that way from Britain.
The routes via Scotland are useful for Scottish trade and trade from the northern end of the North West heading to the north/central parts of the Republic as well as NI, but the bulk of the Republic's population is much closer to Dublin.
So you can narrow those practical backup routes down to Fishguard/Pembroke to Rosslare, Birkenhead to Dublin, Heysham to Warrenpoint, Heysham to Belfast, and Birkenhead to Belfast.
Most of these routes also have a significantly longer overall journey time, so still impact greatly on productivity and costs of haulage for businesses.
Perhaps we should consider funding an alternative berth to be used in emergencies at the Port of Mostyn on Deeside.
It's close to the A55, and would be easy for existing haulage operators to divert to quickly, if there was an incident.
Birkenhead is also useful, but it's already well used, so additional relief sailings have to fit into the existing schedule.
The Port of Mostyn is already well used for maritime operations in the renewable industry, so it wouldn't totally rely on government support for the ongoing maintenance of an emergency berth.