As with anything, generalizations on either side of an argument offer minimal value. It depends on what “irregular” means, and the underlying etiology of the irregularity. Only a doctor–ideally a faithful Catholic doctor–is able to make that determination based on individual patient characteristics (ideally in consultation with a priest and, if necessary, Catholic bioethicist) as to whether NFP is possible after medical treatment.
It’s amazing to see the degree to which there is variability across individuals, even across time, in health needs and compatibility of NFP with various health conditions. I also have a PCOS diagnosis and spent multiple years following the birth of my son in amenorrhea.
Prior to my son’s birth, I cycled irregularly (cycles ranged 24 to 35+ days) and used the symptothermal method to avoid for 5 months before my son was conceived. At the time, I didn’t have a PCOS diagnosis yet. In the four years following his birth, I didn’t cycle once. We used LAM for the first 6 months, then attempted to transition to Symptothermal postpartum rules (which are, effectively, Billings rules with cervical check prior to first ovulation). I had fertile mucus for all but a handful of days during the period up to 2.5 years postpartum, so we had to switch methods. I used Clearblue advanced digital tests as a test of the Marquette postpartum method, but cycled constantly from low to peak, with no high days, still with constant fertile mucus. I later learned that my LH is in a persistent elevated state with PCOS due to my body’s rapid cycling failure to ovulate (due to elevated androgens, verified by serology and an extremely high antral follicle count).
I am currently taking oral contraceptives therapeutically as part of a suite of treatment for PCOS.
This message is meant to convey that different methods will work to different degrees even within an individual over time, and that even with the best efforts and intentions, sometimes medical treatment that contraindicates NFP use is necessary. There are limitations and advantages to each method, and it is largely individual in how those attributes are manifested.