LÉ Emer (P21) was a ship in the Irish Naval Service. The ship was named after Emer, the principal wife of Cúchulainn and the daughter of a chieftain from Rush, County Dublin. After evaluating Deirdre for 3 years, Emer was ordered in 1975. Experience with Deirdre showed a need for more engine power, better crew accommodation and improved silencing. The vessel was modernised throughout her service life in order to take advantage of new technologies and to make life for the crew more comfortable; additions included air conditioning.