I'm gonna be honest with you, considering this is the Leaving Cert I would cover every topic in biology regardless of it being common or not.
I'm sorry to crush your dreams but believe me, covering one or two extra topics is better on the long run than scratching your head during the actual exam. Either way good luck!!
The way the Leaving Cert is structured this year, you can leave out around half of the course and still answer the required number of questions: https://www.folens.ie/leaving-cert-biology-masterclass-lc-2022 . Obviously I wouldn't recommend this, but it means leaving out a few topics is a much safer bet than any other year.