This is Asarina scandens, much admired by students on the home study course at one of the regular garden visits to National Botanic Gardens at Glasnevin in Dublin.
It is a tender perennial herbaceous climbing plant. It always self seeds into the old stone walls at the bots where in trails downwards. It can grow to 2 meters or more. In cold areas it makes an excellent conservatory plant. It is easy to grow from seed.
Ciaran The Gardener 