The enigma surrounding the origins of William John DEMPSEY (1900-1961)

[Dundee]

My great-grandmother Frances Prince (1868-1959) had a sister named Agnes Prince (1872-1940) who married Thomas Foster (1867-1918) and had a son named William (1898-1988) who happened to emigrate to the USA. However, this is not the same individual.....

This is wrong for a start.  Thomas and Agnes' son William was born in 1900 as can be seen in census records and his birth registration, so I agree with Heywood that the descendants of the William FOSTER who emigrated have attached him to the wrong parents.

Debra 

[Dundee]

This William Foster has a birthdate of 19.3 1898
https://www.familysearch.org/ark:/61903/1:1:J6WJ-D6K
His address is 137 Beaufort Street, Liverpool

This is a likely 1911 entry for that William Foster
https://www.familysearch.org/ark:/61903/1:1:XWRB-FTV

Is that tree wrong with the details for William Foster?

That is a crew list for the SS Mauretania.  He had previously served in the RN from 1916 - 1919.  He seems most likely to be the man who married Mabel OTTY and is the illegitimate son of Celia HAMPTON and (maybe) Thomas FOSTER.

If Agnes' son William married Elizabeth HOWARD,  did he die or did he disappear and reinvent himself as William DEMPSEY?  You would need to see her second marriage cert to see how she describes her status.

Debra 

[heywood]

I was thinking from your first post that you definitely knew that Agnes’ son was traced and discounted through research.

I found the marriage to Mabel Otty last night but thought there was enough evidence already to show that it was the wrong William.
However, as Debra mentions it, here you are:
9th February 1921, St Philemon’s, Toxteth Park
William Foster, 22 yrs, Seaman, 52, Elaine St, father, Thomas Foster, Dock labourer
Mabel Victoria Otty, 23 yrs

You can find the couple in Ohio records.

I also came to the same conclusion as Debra, that your William Foster and Elizabeth Howard separated and he sought a new life.

[Dundee]

Great-Grams' sibling Agnes Prince (1872-1940), marr. 1894 to Thomas Foster, basically is my main suspect. She's not just the only full sibling that lived long enough to have a child (she and Thomas had 7 in 1895, 1896, 1898, 1902, 1904, 1907 and 1909)

1895  Thomas
1897  Mary
1898  Margaret (died june qtr 1900 aged 1)
1900  William
1902  George
1904  Agnes
1907  Joseph
1909  John

All registered at Toxteth Park with mother's maiden surname PRINCE.  In 1911 they have 7 living children and one deceased.

Debra 

[heywood]

The child Thomas Foster - possible son of William Foster and Elizabeth Howard is with James and Mary Howard in one record.

Electoral registers 1924 and 1925
38 Robertson Street
James Howard
Mary Howard
Elizabeth Foster

Earlier records just show the Howards but Elizabeth was born 1902 so wouldn’t show.

[heywood]

At 7 Robertson Street, you have Thomas and Agnes Foster in those electoral registers.
1920 and 1921 Thomas, Agnes and William.

[Dundee]

So it's possible some relative actually did order William's marriage certificate, and he had given her real name.

I agree with you there, I think the misspelling of 'Agnes' as 'Agnis' has probably come from a document.

Debra 

[heywood]

*third cousin (3c)
Great-Grams' sibling Agnes Prince (1872-1940), marr. 1894 to Thomas Foster, basically is my main suspect. She's not just the only full sibling that lived long enough to have a child (she and Thomas had 7 in 1895, 1896, 1898, 1902, 1904, 1907 and 1909) but somehow an identical name pops up as William Dempsey's mother in some of Ancestry's trees. And what I've learned from researching the many illegitimate kids in my tree is that they have a tendency to lie about everything except two things: their actual birthday, and the name of their mother. So it's possible some relative actually did order William's marriage certificate, and  he had given her real name.

So how does one go about proving the above theories?

I wonder if you researched William, the son of Agnes or relied on published trees.
In your first post, you write, ‘However this is not the same individual’ and it seems likely now that it is.   

[Dundee]

Just as an aside, Agnes FOSTER born 1904 died unmarried in 1934.  Her address was 35 Barbara Street, the same as her mother and brother John in 1939.  The owner of the plot at Allerton cemetery was her brother Joseph FOSTER and he was buried with her in 1963.

Thomas FOSTER (the father) died in 1916 and is also at Allerton.  His address, 7 Robertson Street.

Debra