Hi

Here's the marriage details for Thomas Keegan to Mary Anne McDermott in Dublin South (same details for both, so a match):

Ireland, Civil Registration Marriages Index, 1845-1958
Name: Thomas Keegan
Date of Registration: Oct-Nov-Dec 1894
Registration district: Dublin South
Volume: 2
Page: 694
FHL Film Number: 101257

From this cert you will get their fathers names, at least, and the church where they were married. The Church cert, if you can get it, may have mothers names.

On Griffiths Valuation, the McDermott name is mentioned 30 times in Queens/Laoise, but with her father's name, you will be able to narrow it down considerably.

Unfortunately, there are 191 mentions of Keegans in Westmeath, so you'd definitely need the name to find Thomas' father!

I wonder if this is Thomas's death info? Details seem (about) right:

Ireland, Civil Registration Deaths Index, 1864-1958
Name: Thomas Keegan
Estimated Birth Year: abt 1865
Date of Registration: Oct-Nov-Dec 1937
Death Age: 72
Registration district: Dublin South
Death Country: Ireland
Volume: 2
Page: 325
FHL Film Number: 101740

And I wonder if this is Bridget Keegan's marriage to James O'Connor (same details for both, so a possible match):

Name: James O'Connor
Date of Registration: Oct-Nov-Dec 1927
Registration district: Dublin South
Volume: 2
Page: 355
FHL Film Number: 101576

There are lots of O'Connor births in South Dublin with mother KEEGAN on ancestry.com, starting with Jeremiah born Sept 1931 (too numerous to name all here, but if you have even 1 name, I can look it up for you).

Would the Mary & Patrick Dempsey that Bridget O'Connor and Mary Anne Keegan are living with in c. 1939 be MARY ANNE KEEGAN??

You say your father Patrick Keegan probably married in the early 1920s. Do you have any idea of his first wife's name? Where he married? Or the names of any of his children from the marriage?

Cheers for now
Colette