Hi Jane
I might have made progress in establishing the parents of your William Killey b Santon circa 1815.
I tried my new favourite facility, Ancestry photos, and put in "Killey" and "Isle of Man", and up came an entry for an Isabella Killey "b 1876 Bride" with parents "Phillip Killey 1821- " and "Catherine 1815-", showing that the submitter hadn't researched the parents of Isabella other that seeing their names on census entries.
The beauty of this facility is that the census pages are there for you to click on, so here are transcriptions of 1881 and 1891:
1881 Bride district 1B, page 2
Phillip Killey head age 60 b Santon
Catherine wife age 66 b Andreas
Catherine Gale stepdau age 16 b Jurby
Jane dau age 13 b Bride
Eliz dau age 7 b Bride
Isabella dau age 5 b Bride
Phillip son age 2 b Bride
1891 Kirk Bride district 1c, page 8
Catherine Killey age 55 b Andreas
Isabella dau age 14 b Bride
Phillip H son age 13 b Bride
Wm Edward Criggal grandson age 7 b Bride
Catherine turns out to be Cath Gale widow, nee Cornish or Comish, probably. She married Phillip on 8 March 1868 Bride as Cath Comish. This needs checking.
The birth parish of Phillip sr in 1881 rang a bell, so I looked at the siblings of the William Killey b 1814 Santon who might be your gg (g?) grandfather:
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Results for: Any Event, 1803 - 1823, Isle of Man, British Isles
Father: John Killey, Mother: Mary Callow
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International Genealogical Index / British Isles - 13
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1. PHILIP KILLEY - International Genealogical Index
Gender: Male Christening: 17 MAR 1816 Santon, Isle Of Man, England
2. JOHN KILLEY - International Genealogical Index
Gender: Male Christening: 01 JUL 1810 Santon, Isle Of Man, England
3. WILLIAM KELLEY - International Genealogical Index
Gender: Male Christening: 15 MAR 1814 Santon, Isle Of Man, England
4. MARY KILLEY - International Genealogical Index
Gender: Female Christening: 27 MAR 1812 Santon, Isle Of Man, England
5. Ann Killey - International Genealogical Index
Gender: Female Christening: 12 APR 1805 Kirk German, Isle Of Man, England
6. MARGRAT KILLEY - International Genealogical Index
Gender: Female Christening: 16 OCT 1808 Patrick, Isle Of Man, England
7. ANN KELLEY - International Genealogical Index
Gender: Female Christening: 12 APR 1805 Kirk German, Isle Of Man, England
8. ELINOR KILLEY - International Genealogical Index
Gender: Female Christening: 09 AUG 1818 Kirk German, Isle Of Man, England
12. Ruth Killey - International Genealogical Index
Gender: Female Christening: 15 MAR 1821 Kirk German, Isle Of Man, England (patron sub)
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It looks as though, after their marriage on 20 Oct 1804. John Killey and Mary Callow had Ann 1805 and Margaret 1808 in Patrick / German, and then moved to Santon where they had John 1810, Mary 1812, William 1814, and Phillip 1816. Then they moved back to German and had Eleanor in 1818, and possibly Ruth in 1821.
In searching for Phillip prior to 1881, I discovered that there were very few Killey males born in Santon between 1805 and 1825, and I suspect that the only couple who did have boys were John Killey and Mary Callow.
The Phillip who fits the bill for "father of Isabella b 1876" turns out to be Phillip bap 17 Mar 1816 Santon. Here he is in 41, 51 and 71:
1841 Bride district 1, page 6
Ballakilley cottage
Phillip Killey age 25 ag lab
Margaret age 25 (I think this is Mgt nee Quayle)
William age 4
Margaret age 1
1851 Bride district 1, page 8
Phillip Killea head age 35 farm lab born Santon
Margaret wife age 35 born Maughold
William son age 14
Mgt dau age 10
Christian dau age 8
Jane dau age 2
children b Bride
1861 can't find
1871 Bride district 1, page 2
Phillip Killey head age 52 farm lab born Santon
Catherine wife age 30 born Andreas
Catherine Gale stepdau age 6 b Andreas
Jane Killey dau age 2 b Bride
The other Santon born male Killey from circa 1813 is John. He too was living in the north of the island. He married Jane Hudgin 11 Aug 1832 Jurby:
1841 Jurby district 2. page 3
John Killey age 30 ag lab
Jane age 25
Cath age 8
John age 6
William age 5
Jane age 1
1851 Jurby district 3, page 3
Ann Hutchin head widow age 73 labourer b Malew
John Killey son-in-law age 39 ag lab b Patrick
Jane Killey dau age 37 b Arbory
John gson age 16 ag lab b Jurby
William gson age 14 scholar b Jurby
Ann gdau age 7 b Jurby
Thos gson age 3 b Jurby
1861 Jurby district 4, page 4
John Killey head age 50 ag lab born Santon
Jane Killey wife age 47 dressmaker born Rushen
Thomas Killey son age 13 born Jurby
Eliza J Curphey gdau age 2 born Jurby
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Results for: Any Event, 1828 - 1868, Isle of Man, British Isles
Father: John Killey, Mother: Jane Hudgeon
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International Genealogical Index / British Isles - 7
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1. WILLIAM KILLEY - International Genealogical Index
Gender: Male Christening: 17 APR 1836 Jurby, Isle Of Man, England
2. ROBERT KILLEY - International Genealogical Index
Gender: Male Death: 13 DEC 1857
3. ROBERT KILLEY - International Genealogical Index
Gender: Male Christening: 31 JUL 1857 Jurby, Isle Of Man, England
4. CATHARINE KILLEY - International Genealogical Index
Gender: Female Christening: 31 MAR 1833 Jurby, Isle Of Man, England
5. ANNE KILLEY - International Genealogical Index
Gender: Female Christening: 31 MAR 1844 Jurby, Isle Of Man, England
6. JANE KILLEY - International Genealogical Index
Gender: Female Christening: 03 MAY 1840 Jurby, Isle Of Man, England
7. JOHN KILLEY - International Genealogical Index
Gender: Male Christening: 23 NOV 1834 Jurby, Isle Of Man, England
There's also a Thomas Killey bap 5 March 1848 Jurby, to John Killey and Jane Hugdin who took more finding.
If you look at each IOM census in turn, and input only "Killey" , birth year "1815 plus or minus 5 years" and birth parish "Santon" and nothing else, you'll see how few there were. This is why I have a strong hunch that your William was the son of John Killey and Mary Callow. Now you need to look at film of marriages and baptisms.
The other good thing is that if you can demonstrate that the John, William and Phillip outlined above were brothers, and sons of JK and MC, then you've got a contact on the photos site, and a pic of a first cousin of your William.
Jean C