**Think of the advantages of that and the ferry service from Ireland to France, France to mainland Europe**
France and Ireland have a hard border for non EU citizens.
Ireland and UK are in the Common Travel area and neither are in Schengen. So an Irish or UK passport holder can move freely between ROI and UK. Ireland is in the EU, so an Irish passport holder can move freely between ROI and EU. As UK passport holders can enter Schengen for 90 days, they can move from ROI to Schengen in the same way as from UK to Schengen (using a passport).
Non-EU people must present a passport at the ROI-Schengen border so an Indian or Chinese national can enter UK with a UK visa and then travel to ROI due to the CTA, but this doesn't permit entry to Schengen so they will be rejected at the border with France.
EU will know about time spent in Schengen from some time in 2024 when the Big Brother EES system starts, so there will be no way to get around the Schengen's "90 days in 180 days" rule (unless assisted by Wagner Group or similar to evade border controls). I expect calls for a Schengen-wide tax residence rule to follow EES implementation, to prevent nomad-like living in Schengen.
ETIAS will probably go live in 2025. Even UK passport holders will need to pre-apply and pay €7 for ETIAS (similar to US ESTA) to travel from ROI to France, unless a beneficiary of the Withdrawal Agreement.