Irish dairy farming is characterized by a predominantly spring calving, pasture based production system with strategic use of concentrate supplementation when pasture availability is less than herd demand. Cereal grains are used extensively in concentrates as an energy source, but the use of these human edible energy sources is becoming somewhat controversial. By contrast, by-products, which cannot be consumed by humans, such as maize dried distillers’ grains (DDG), palm kernel expeller meal (PKE) and soybean hulls (SH) can provide an economical source of nutrients to complement grazed grass. While these products have been extensively studied in US and New Zealand production systems, there is a paucity of information on their use in moderately yielding (30 kg/d) pasture fed cows. The objective of this research was to evaluate the effect of replacing barley with increasing dietary levels of by-products (DDG, PKE and SH) on milk production and composition, pasture and total dry matter intake (DMI), body condition score (BCS) and body weight (BW) and nitrogen (N) excretion in early lactation dairy cows grazing perennial ryegrass-based pasture. Forty-eight Holstein Friesian dairy cows (64 DIM ± 24) were blocked by pre-experimental milk yield and offered 1 of 4 supplementary concentrates (5.17 kg of DM/d) containing 35, 55, 75 and 95% by-products for T1, T2, T3 and T4 respectively. The concentrates offered contained DDG, PKE and SH in equal portions and the experiment continued for 70 d. Data were tested using Proc Mixed of SAS with trt, week, parity and their interactions included as fixed effects in the model. Treatment had no effect on milk yield (30.8 kg/d; P = 0.78) or fat and protein yield (2.1 kg/d; P = 0.57). Similarly, BCS (2.75; P = 0.76) and BCS change (−0.05; P = 0.74) were not affected by treatment. There was also no effect of treatment on pasture DMI (15.73 kg of DM/cow/d; P = 0.99), total DMI (21.05 kg of DM/cow/d; P = 0.99), digestibility of NDF (0.69; P = 0.34), ADF (0.59; P = 0.36) or N (0.33; P = 0.70) or the excretion of N in urine (0.26 kg/d; P = 0.99). The results from this research show that cereals can be replaced with by-products (DDG, PKE and SH) in the diet of grazing dairy cows without any adverse effects on production, feed intake, digestibility, BW, BCS or N excretion.