120 lights around the coast and up the rivers, oldest first. The earliest date of any light still in service is Hook Lighthouse: 1172.
Of the lights still in service in this list, the earliest date belongs to Hook Lighthouse at Hook Head: 1172. The list gives one year per light, which for some is when the present tower was built and for others when a light was first kept there.
Most are maintained by the Commissioners of Irish Lights, which covers the whole island; 24 in this list are kept by harbour and port authorities, and 21 are no longer lit.
The list and years come from the tables in Wikipedia's List of lighthouses in Ireland ↗, which draws on Russ Rowlett's Lighthouse Directory and the Admiralty List of Lights. The list gives one year per light: for some it is when the present tower was built, for others when a light was first kept on the site, so check the light's own article before quoting it. Where the table gives none, a dash is shown. The Commissioners of Irish Lights publish their own details at irishlights.ie ↗.