The binaries available on the apt source provided by flirc are only available for ARM 32bit. This will work on the ARM 64bit architecture, but the libreadline dependency will not be installed by APT. I solved this by adding armhf to my architecture list, installing the 32bit version of libreadline, and linking it to libreadline.so.6 so flirc can find the dependency. Ideally flirc would release a binary for arm64 so it works with modern raspberry pi architectures...
sudo apt install curl gnupg
curl https://apt.fury.io/flirc/gpg.key | sudo apt-key add -
echo "deb [arch=armhf] https://apt.fury.io/flirc/ * *" | sudo tee /etc/apt/sources.list.d/fury.list
sudo apt install flirc
sudo dpkg --add-architecture armhf
sudo apt install libreadline8:armhf
cd /lib/arm-linux-gnueabihf/; sudo ln -s libreadline.so.8 libreadline.so.6