Irish dairy production is based on a spring calving grazing system with the use of CS when there is a shortage of grass. Milk quality and processability deteriorates in late lactation and the issue is more pronounced in seasonal production systems. Genetics and supplementation strategies may affect MQ and MP. Therefore, the objective of this study was to investigate the effect of milk PTA, CS level and their interaction on late lactation MQ (composition) and selected MP (rennet coagulation time (RCT), ethanol stability (ES) and milk pH) parameters. A factorial experiment, with 2 milk PTA groups [high (+191 kg) versus low (−53kg)] and 2 CS levels (0 (Grass-only; GO) versus 2.7 kg DM CS) was carried out over a 51-d period. Forty-eight Holstein Friesian (24 low milk (LM) and 24 high milk (HM) PTA) dairy cows were blocked on parity and balanced on DIM, BCS and Economic Breeding Index. Cows within PTA groups were randomly assigned to one of 2 CS levels, resulting in 4 treatment groups (n = 12): 1) LM PTA offered GO (LG); 2) LM PTA plus 2.7 kg DM CS (LC); 3) HM PTA offered GO (HG) and 4) HM PTA plus 2.7 kg DM CS (HC). All cows were grazing full time and were offered 17 kg DM grass/cow per d. There was no interaction for any parameter measured. Milk pH, ES and RCT did not differ between treatment groups. However, cows offered CS had higher ES (+2.07%, P < 0.05) than cows offered GO. There were no milk compositional differences between LG and LC. Casein % was higher in HG (+0.15%, P < 0.05) compared with HC and was also higher in LC (+0.15%, P < 0.05) compared with HC. Fat % tended to be higher in LG (+0.21 kg, P = 0.06) compared with HG. Cows in the HM PTA group had decreased fat % (−0.22%, P < 0.01), protein % (−0.12%, P < 0.05) and casein % (−0.11%, P < 0.01), while CS also reduced fat % (−0.17%, P < 0.05), protein % (−0.14%, P < 0.05) and casein % (−0.11%, P < 0.01). In conclusion, CS and HM PTA negatively affected milk composition. Offering CS increased ES suggesting CS may increase colloidal stability of milk.