Steps for configuring raspberry-pi

apt update && apt upgrade -y
apt install lirc
cd /etc/lirc/lircd.conf.d/
wget -O lircd_celsius.conf https://github.com/mattjm/Fujitsu_IR/raw/master/lircd_celcius.conf  # for Fujitsu AC

Configuration for configuring pi at boot (kernel modules);

dtoverlay=gpio-ir,gpio_pin=18,gpio_pull=true
dtoverlay=gpio-ir-tx,gpio_pin=27

At this point, you can reboot to configure the GPIO pins. The next part is to mofidy the lirc_options.conf file to use predictive device name and kernel.

driver          = devinput
#driver          = default
#device          = auto
device          = /dev/lirc0

In theory, shouldn't need to modify the device nor the driver, but I've found it might help.

The reason why this is theory only, is that we will use the following command ;

irsend SEND_ONCE fujitsu_heat_ac heat-high-24.5C

which should be using the right device right away, but in the case it doesn't find the right transmitter GPIO, just find out which lirc device is it (dmesg |grep -i ir, rc0 is lirc0).