A tiny pup was saved from freezing to death by a young man who found her in a black plastic bin bag on his way from a St Patrick's Day disco.

The man was walking home through Roscommon town at about 5am when "he could see something wriggling in a plastic bin bag and there were sounds coming from it".

"The pup had been put in the bag and pegged out and onto a laneway," said Billy Gallagher, welfare officer with Athlone SPCA. "When this young man bent down to see what was in the bag, she hopped out.

"She was shivering from the cold and it was lashing rain."

Billy said that the pup, now named Maisy, was most likely dumped by a small-time puppy farmer. She appears to be a cross between a terrier and a chihuahua.

"He saved her life. If she had managed to wriggle out of the bag herself, she was beside a busy road."