Peter
Many thanks for reply. I agree with your facts regarding Ann C's 1851 census. There is no other Ann Cretney of the correct age so she must be the Catherine Ann Cretney b 8th Sep 1833 known as Ann daughter of Thomas C and Ann Clucas.
The only doubt I have is Thomas's Will (copy below) written in 1858 and proved in 1863 so his death on 22 May 1861 ties in. In the Will he leaves 2/6 to his daughter Ann Cretney, but she would be married by then.
William her brother married Mary Killey on 23 Nov 1861 is the Mary living at K Michael in the 1871 census.
Hope this helps your searches.
Allan Cretney

Will: Thomas Cretney 1863 - 38 Film GL773
In the name of God Amen. I Thomas Cretney of Ballagary in the parish of Marown being of sound mind memory and understanding do make, publish and declare this to be my last Will and Testament in way and manner following:-
Firstly I command my sole to Almighty God and my body to a Christian Burial.
Second, I leave and bequeath to my beloved wife Ann Cretney the whole of my share of the money that was left me by my brother William Cretney arising from a sale of the said William Cretney’s property called and known by the name of Cretney’s Chibanagh and also my share of the rents of the said property up to the twelfth day of November last past and also my share of his goods and effects.
I leave and bequeath to my five sons namely John Cretney, William Cretney, Robert Cretney, Philip Cretney and Edward Cretney the sum of two shillings and sixpence each.
I leave and bequeath to my two daughters namely Ann Cretney and Jane Cretney the sum of two shillings and sixpence each.
I nominate my beloved wife Ann Cretney aforesaid my whole and sole executrix of the whole of my goods and chattels and effects may they be of what nature whatsoever. As witness my mark to my name this twenty sixth day of November one thousand eight hundred and fifty eight 1858.
Signed in presence of the testator who at his own request and in own presence has affixed his mark to his name this day. Thomas Cretney his X mark. Cleave Kinley, James Cretney his X mark.

In the Worship Richard Duff esquire Vicar General of this Island do a. The humble petition of Ann Cretney of the Parish of Braddan Widow shareth That your petitioners husband Thomas Cretney sometime since departed this life having first duly made and published his last Will and Testament in writing where of he appointed your petitioner executrix. That your petitioner is desirous of removing the executrix of the said Will and it is accepted that some fit and proper person should be appointed and sworn administrator of the estate and effects of the said Thomas Cretney.
Whereas the petitioner humbly prays a hearing of this petition and that your Worship shall be pleased to grant administration of the estate and effects of the said Thomas Cretney to some fit and proper person and petitioner will pray. James Spittall advocate for petitioner. Heard Douglas 22nd may 1863 and accepted.