The following excerpt is from the Ask About Ireland website's page on roof thatching in Ireland.
Traditional Irish houses are generally rectangular in layout. This design requires simple roofing formats, either gable-roofs or hip-roofs. The gable-roof has two surfaces, front and back, whereas the hip-roof has four roof surfaces, which include the slopes at either end; these styles tended to vary from region to region.
In past times, the general roof covering of choice and economic necessity has been the thatched roof. Reasons for this probably relate to the fact that the organic matter of a thatched roof tended to be available locally and at much less expense than solid roofing materials.
You can read more by visiting the page on the Ask About Ireland dot ie website. The page contains interesting info and images covering roof thatching in Ireland.