Since our departure from the EU was in early 2020, travel has been rather limited but I have some experience of France and Ireland.
France, to which I have travelled by 'chunnel' is much the same as it always was, perhaps a slightly longer queue in Folkestone when heading to the continent, but arriving on the French side there is literally no interaction with border control (straight onto the autoroutes). Coming in the other direction it's pretty smooth.
I haven't as much experience with the ferries post-Brexit as I do before, but delays and the like are well known about at Dover. Arriving into Portsmouth from Ouistreham (especially on the daytime sailings) always seemed to be a good candidate for long delays & queues, I don't suspect this has improved since Brexit.
Anyone wishing to cross the channel (and avoid the hassle of the 'big' ports at Dover and Portsmouth) should try the DFDS/Transmanche Newhaven-Dieppe sailing, 4/5 hours and it's usually pretty quiet. I can't comment on the experience post-Brexit, but when I last sailed that route in 2018, there weren't really any queues to speak of at either end, nor on the return.
Ireland is much the same as it was before. Ferries first, the Holyhead-Dublin route operates as normal, with little to no effort put in to extra checks (even passenger locator forms, when they were a thing). This goes for both Irish & Stena. I have yet to try it but I hear that the Pembroke & Fishguard-Rosslare routes are much the same, if quieter.
Flying into Dublin takes a bit of getting used to, it's often very bad for long waits on both arrival and departure. Interestingly, on arrival at Dublin from the UK, most, if not all of the passengers head straight for the 'non-EU' passport gates, when in fact UK passports will work the EU gates. (Mine did and it's one of the new 'bloo' ones.)
The border between the North and the Republic remains fully open, so anyone wishing to avoid a bit of potential hassle can fly to Belfast and drive across, or alternatively take the Birkenhead-Belfast ferry. I've tried both since Brexit and both are hassle free, being a domestic journey.
I'll fly to Portugal at the end of this month, so I'll update this post accordingly.