We need to separate the immigration issues (people) from the customs issues (goods).
The EU has freedom of movement. So does the CTA. This means that people have the right to travel (although not necessarily the right to travel without being checked).
Schengen is a common area for immigration purposes. This means that people can, in practice, travel without being checked.
The EU has free movement of goods. Goods (and that includes parcels, boot-loads of wine, trucks carrying food, things carried on aeroplanes and so on) are not checked when moving between countries.
After Brexit, the UK will still be a member of the CTA, so people can move between the UK and Ireland but will still need to be checked as per CTA rules (GAR forms etc.).
The UK will not be a member of the EU, so people will not have an automatic right to travel to the EU. However, it is possible/likely that there will be reciprocal visa-waiver arrangements in place and that most people won't notice any difference in practice.
The big change will be that ALL goods will need to be checked. This is a pretty hard fact unless some a transitional agreement is put in place that keeps the UK inside the customs union.
This means that anyone getting off the ferry at Calais, Dover, Fishguard, Rosslare etc. will need to take all of their goods through customs checks. I don't think light aviation can honestly expect any alleviation from the same set of rules.
The result may well be that we need to fly to and from a customs designated airfield on trips between the UK and Ireland and the UK and the continent. This will be a huge restriction as many of the current "immigration" designated airfields are not "customs" designated.
The RYA has some excellent guidance here:
https://www.rya.org.uk/knowledge-advice ... rexit.aspx