The importance of understanding shopping behaviour of consumers cannot be underestimated. The purpose of this study is to identify the main determinants and motivational factors that differentiate Generation Z (Age:18-24) and Generation Y (Age:25-34) consumer groups in Ireland and how this influences their behaviour when shopping for clothes with online retailers. A descriptive research design along with deductive positivism philosophy was applied. Primary data was collected through an online survey of 87 consumers who have experience in purchasing clothes online. Findings show that the most pronounced differences between Generations Z and Y relate to hedonic motivation of Generation Z and utilitarian motivation of Generation Y along with preferences of social media platforms, mobile retailer apps, discounts, uniqueness of clothes and tendencies to purchase from one or more brands. These findings are important for online fashion retailers to understand as these generations represent the largest consumer groups who purchase clothes online.