I have dual citizenship - Irish/British - and have passports for both countries.
Am I bound by the 90/180 day rule when travelling in the EU after Brexit?
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Because of your dual citizenship, you are in the position of being able to effectively ignore the border control aspects of Brexit. No matter what happens, you will still have the right to enter both the UK and the EU without restrictions.
The only thing that could change this is if Ireland chooses to leave the EU, too. This does not seem likely at this point.
Even after Brexit, you will be an EU citizen because of your Irish citizenship, so you have all the rights that an EU citizen has. When you enter the EU, you should do so using your Irish passport. There are more details in the answers to our reference question about using two passports.