OK, i have done some research on this, including a couple of calls to labs offering PCR tests in Brittany, - and have pretty much answered my own question, but it may have relevance for anyone planning to go to France in the summer with unvaccinated children.
The labs generally can provide a result within 48hrs in slightly above 90% of cases. Quite often it will be available the next day, but you cannot depend on it. If they have symptomatic local cases, they will take priority and they will not guarantee they wil do your test on the day you book, but they reckon that should be rare by summer. Last summer the French government supported local labs to be set up in areas of mass tourism, to add capacity for travellers. No one knows if that will happen again in 2021.
If you get a test done at the last possible moment (to give yourself the best chance of having a valid result by the time your arrrive at an Irish port), your margin for error is approximately 6 hours. i.e. if the ferry were 6 hours late, according to current rules you would not be allowed to disembark. If you get your test done a little too early, this erodes into the safety margin. Flights do not have a problem really (2 - 4 hours flying time increase your safety margin from receiving your PCR result to arriving in Ireland by several hours).
This issue has not been road-tested yet, and probably few people are aware of it, because non-essential France-Ireland travel has not occured under these rules. Basically unless the Irish authorities change their stance, there is no practical way that I can see for Irish families (with unvaccinated children) to travel by ferry to France this summer. The problem is only on the return journey. A change in the rule to a negative test 72hours within embarcation, rather than within 72hours at time of arrival, would be enough to greatly facilitate things.