The Irfon defines the northern limit of the Mynydd Epynt area between Llanwrtyd Wells and Builth Wells . Irfon defines the northern border of the Mynydd Epynt area between Llanwrtyd Wells and Builth Wells . The Irfon defines the northern border of Builth - Wells - area between Llanwrtyd - Wells and Mynydd - Epynt .