The man is leading the horse by a kind of leash. This is a regular scene of the road around here, related to the presence of an Irish Traveller community.
Irish Travellers are a distinct minority ethnic group in Ireland, with specific genetic markers. They maintain an itinerant and precarious way of life, and proprietary traditions that include a great care for horses. They are believed to be descendants from the Irish settlers who lost their home during the Cromwellian conquest of Ireland in the 17th century.