Emissions from real-world driving are often higher than estimated emission levels based on Euro emissions standards and type-approval tests. Consequently, the air quality in cities has not improved as originally anticipated from fleet renewal and stringent emission regulations for newer vehicles. This paper presents the real-world emission factors (EFs) for in-use cars, taxis, vans, and buses obtained using on-road remote sensing in Dublin city, Ireland. EFs from both petrol and diesel Euro 6 vehicles were significantly improved compared to the EFs of older emission standard vehicles. The analyses indicated that the presence of a few high emitting older vehicles skewed the overall distribution of EFs. Detailed description of variations in EFs for individual pollutants based on changes in emission standards, location, and other driving characteristics were studied and reported, which provide justification for implementing policies targeting fleet renewal.