Thanks for all the help. As I live in South Australia I've done some more digging. According to a newspaper marriage announcement Sarah Cox Looney was the adopted daughter of John Looney:
MICHELMORE—LOONEY.—On the 12th June, at the residence of Mr. William Mudge, Maclaren Vale, by the Rev. T. Piper, Charles Andrew, second surviving son of the late Mr. William Michelmore, of the Meadows, to Sarah, adopted daughter of the late Mr. John Looney, of Sea View, near Maclaren Vale.
My guess is that John might have married Sarah Strong to get her out of a predicament.
Rachel is buried at Willunga and her headstone made of Willunga slate is still in excellent condition.
If anyone is interested I've found Rachel Redhead Looney's arrival in South Australia was on the ship Glenarm which departed Liverpool 3 Aug 1842 and arrived in Adelaide on 1 Dec 1842. (SA Shipping Register)

Thanks for your comments with regard to this matter.
Margaret