Rose Henderson appeared as Sister Assumpta in two episodes of Father Ted: 

“Cigarettes And Alcohol And Rollerblading” Episode 8 (26 APR 96)

It’s Lent and Father Ted has decided to give up smoking, Father Dougal has given up rollerblading and Father Jack has been forced, against his will, to give up the drink. Their sacrifices prove to be diffcult to cope with, so Ted calls in Sister Assumpta to supervise their self-sacrifice. She proves to be an out-and-out sadist but when they catch her eating their chocolate Easter eggs, they get rid of her by sending her to sort out their rivals on Rugged Island.

And God Created Woman Episode 5 (19 MAY 95)

Polly Clarke, a famous erotic novelist, (played by Gemma Craven) pays a visit to Craggy Island. Father Ted gets the (mistaken) impression that she has taken a fancy to him. Sister Assumpta and her group of nuns take action to ensure that Ted and Polly are kept apart.  Sadly his 30 second mass is a huge disappointment and they manage to enlist Polly Clarke to join their ranks.