Visit to the UK with infant (non-EU passport)

Hi!!

Apologies if something similar has already been addressed but I've yet to find it.

Infant's mother Irish, infant's father non-EU. All residing in another EU member state (jus soli not an option). Due to the fact that the infant was born outside of Ireland the child must be registered as a foreign birth before being able to get a passport - due to the pandemic there's a back log of two years. Infant has father's nationality currently and EU family member residents card.

Since brexit I'm aware non EU family members(article 10 residency card holders) aren't able to visit the UK without a visitors visa (unless I'm misinformed); I'd like to ask for confirmation if this still applies for an infant?
Or can the Irish mother can prove at the border the duration of the short stay, ties to the country of residence (home, job, etc.), and can provide documents such as birth certificate and confirmation of Irish registration application?

Thank you in advance!!

How does the family plan to travel to the UK?

If they are planning to fly to the UK, keep in mind that they may not even get a chance to make their case to the Border Force. The baby may not be allowed to board the flight without a valid visa to the UK by airline staff. And I doubt that the parents would want to travel without the baby.

Was the mother Irish by birth in Ireland or was she born outside Ireland?