Reproductive efficiency in the Irish beef cow population is challenged by age at first calving, long calving intervals and limited AI usage. Low average herd size and often part-time nature of beef production increases the challenge of detecting cows in estrus. The objective was to establish an estrous synchronization regimen to facilitate fixed-time AI (TAI) in beef cows under Irish conditions. A total of 1410 suckled cows located on 61 farms were enrolled across 2 replicates (Spring, n = 703 and Autumn, n = 707). Presence of a corpus luteum (CL) and body condition score (BCS) were recorded at treatment initiation. Following a voluntary waiting period of 35 d postpartum, cows were assigned to (1) receive progesterone-releasing intravaginal device (PRID Delta) on d 0 and PRID removal on d 7 with an 25 mg injection of PF2α (Enzaprost) (Group 1); (2) same as Group 1, but cows received a 100 ug of GnRH analog (Ovarelin) on d 0 (Group 2); (3) same as Group 2, but cows received 400 IU equine chorionic gonadotropin (eCG) (Syncrostim) on d7; (Group 3). All cows received GnRH at TAI 72 h after PRID removal with pregnancy diagnosis by transrectal ultrasonography 35–40 d later. Data were analyzed using the GENMOD procedure of SAS. Presence of a CL (Spring 50.6%; Autumn 70.0%) increased pregnancy rate independent of treatment and season (P = 0.03). Mean BCS in Spring and Autumn was not different but BCS positively affected pregnancy rate independent of treatment or season (P = 0.003). There was a significant treatment × season interaction for pregnancy rate (P = 0.0002). In spring, overall pregnancy rate was 59.1% (416/703) and was affected by treatment (49.6 vs. 59.3 vs. 68.5%, for Groups 1, 2 and 3, respectively P < 0.05). In contrast, in autumn, overall pregnancy rate (52.6%, 364/707) was unaffected by treatment (53.7 v 52.0 v 48.7%, respectively). In conclusion, GnRH at the initiation of a TAI synchronization protocol and eCG both increased pregnancy rate in spring-calving suckled beef cows. This effect was not evident in autumn-calving cows. Seasonal differences in outcome may reflect differences in management (grazing vs. confinement), days postpartum or breed type and remain to be elucidated.