I don’t know about entry to Dublin, but if it isn’t a red list country you are coming from then you would only need to isolate at a place of your choosing, no one will be interested in your subsequent travel arrangements at your time of arrival.
from Dublin to the UK, you need to complete a plf if you have been outside the CTA within the last 10 days. If you’ve been in a red country then only certain airports are allowed as entry points and you need to go straight to the quarantine hotel for 10 days. There is no exception for family illnesses.