Seanán means the old wise man or a young person who is wise like an old person.
Sean [pronounced shan] is the Irish word for old. The diminutive án in Irish means a thing or a person. It is found in many Irish male names including Ciarán, Rónán, Ruán, Colmán etc. Án is pronounced like awn. Together, they are pronounced shan-awn.
It is an exclusively Irish name and is found in county Clare/An Clár in particular.