In the spirit of starting with what you know, here's what I did:
To attempt to find an estimated birth year for Florence, I looked up David Morgan in the Social Security Death Index. Here he is:
David Morgan, b 2 Jun 1887, d. Dec 1968; last residence Scranton, Lackawanna County, Pennsylvania.
In an effort to pin down when Florence may have married David, I needed to first know when he came to the U.S. I found his passport application, which states:
He was born 27 Nov 1888 [off from the SSDI] in Aberdale, Wales, son of Howell T. Morgan, and that David Morgan emigrated to the U.S. 6 June 1907. That since that time and before applying (27 Dec 1924) for the passport, he'd lived in Philadelphia, Wilkes-Barre, Detroit, then Scranton. He became a naturalized citizen on 2 Dec 1915. He'd been back to England a couple of times (and when and for how long). Oh, and on the back there's a photograph of him!
UPDATE: Not sure the above man is him. Might be one of the other men by this name.
I then looked for him in Scranton in the 1930 census. There he is, 42, married to (presumably an earlier) wife Sylvia, 32, and living with them is his sister Ann, single, 27. David was a coal miner.
In 1940, still in Scranton, 52, still with Sylvia, 42. No sister. No children.
Therefore, he married Florence after 1940.
That means that Florence is likely to be a Gallagher in 1940.
UPDATE: strike-through language above: WRONG man. Disregard. Sorry about that.
I'll look for her first in/near Scranton. Oh dear. Popular name. No Florence Gallaghers in Lackawanna County, but many throughout the state of Pennsylvania. Did a quick scan for Florence Gallaghers with daughter named Margaret, no initial luck, but only checked six. Also, depending on when Margaret was born, she might no longer be with her mother in 1940.
So we need a bit more information, anonymous. First name of Gallagher husband? Approximate birth year/place of daughter Margaret?