Cellach (hypocoristic Cellachán) is an Irish name. It might refer to:
- Cellach I of Cennrígmonaid, a 9th/10th century bishop
- Cellach II of Cennrígmonaid, a 10th century bishop
- Cellach mac Máele Coba, a 7th century High King of Ireland
- Cellach mac Fáelchair, king of Osraige
- Cellach mac Rogallaig (d. 705), a king of Connacht
- Cellach Cualann, a 7th/8th century king of Leinster
- Cellach mac Dúnchada, king of Leinster
- Cellach mac Brain, king of Leinster
- Cellach mac Faelan, king of Leinster
- Cellachán Caisil, a 10th century king of Munster
- Cellach húa Rúanada, Irish poet (d. 1079, Annals of Ulster)
- Saint Cellach, 11th/12th century bishop of Armagh
- The Cailleach, a figure from Irish and Scottish mythology