A set of bitumen samples from the Carboniferous of Ireland shows a general trend of increasing maturity southwards across the island. This trend is consistent with a similar southwards increase in maturity exhibited by the host rocks (vitrinite reflectance), reflecting proximity to the Hercynian Orogen to the south of Ireland. Increasing maturity is reflected by decreasing H/C ratio, and increasing reflectance, bireflectance and anisotropy, and is accompanied by heavier carbon isotope composition for the most mature samples. Bitumen reflectance is the most sensitive parameter and ranges up to 3.6%. Most of the bitumens occur in hydrothermal mineral veins and were generated locally by the passage of hot fluids through Carboniferous shales and limestones. Some bitumens contain inclusions of metalliferous and other inorganic minerals. High sedimentation rates during the Carboniferous caused hydrocarbon generation before the Late Carboniferous Hercynian Orogeny, so that the bitumens experienced thermal overprinting by the Orogeny and show the same maturity trend as the country rocks. Anisotropy is exhibited by all bitumen samples with reflectances of 2.0% or over; mesophase textures occur in bitumens with reflectances over 3.0%, including one case where fluid inclusion homogenization temperatures in coeval minerals (calcite and fluorite) are less than 200 degrees C.