Irwin was born in London, England, and educated at Clifton High School in Bristol, and at Oxford University. She began writing books and short stories in the early 1920s. She married children's author and illustrator John Robert Monsell in 1929.
Her novels were esteemed for the accuracy of their historical research, and she became a noted authority on the Elizabethan and early Stewart era. One of her novels, Young Bess about the early years of Queen Elizabeth I, was made into a movie starring Jean Simmons.