I have three Thornhill daughters arriving in Victoria between 1857 and 1859. Their names are Frances (Fanny), Elizabeth and Margaret. They were the daughters of Elizabeth Thornhill and William Dudley.
All were baptised in Mitchelstown County Cork Ireland in the 1830s. Elizabeth snr is my "Brick wall". I can find no marriage for her and therefore no parents, birthdate or place.
She died in Mitchelstown in 1845, leaving seven young children. The names of the others are William, George, Christopher and Susan. A family story states that she was a Quaker.