Irish Sign Language (ISL, Irish: Teanga Chomharthaíochta na hÉireann) is the sign language of Ireland, used primarily in the Republic of Ireland. It is also used in Northern Ireland, though British Sign Language (BSL) is also used there. Irish Sign Language is more closely related to French Sign Language^{[clarification needed]} than to British Sign Language, which was first used in Dublin, Ireland. It has influenced sign languages in Australia and South Africa, and has little relation to either spoken Irish or English.

A sign language (also signed language) is a language which, instead of acoustically conveyed sound patterns, uses manual communication and body language to convey meaning. This can involve simultaneously combining hand shapes, orientation and movement of the hands, arms or body, and facial expressions to fluidly express a speaker's thoughts.

Wherever communities of deaf people exist, sign languages develop. While they utilize space for grammar in a way that spoken languages do not, sign languages exhibit the same linguistic properties and use the same language faculty as do spoken languages. Hundreds of sign languages are in use around the world and are at the cores of local deaf cultures. Some sign languages have obtained some form of legal recognition, while others have no status at all.

One of the earliest written records of a signed language occurred in the fifth century BC, in Plato's Cratylus, where Socrates says: "If we hadn't a voice or a tongue, and wanted to express things to one another, wouldn't we try to make signs by moving our hands, head, and the rest of our body, just as dumb people do at present?" It seems that groups of deaf people have used signed languages throughout history.