Thank you Elwyn and Margot.
I agree that it is probably too much of a coincidence that the Sarah and Teresa Dawson mentioned in that 1911 census, is not the Sarah & Teresa mentioned on the marriage record.
The Eliza Street address though relates to Frederick's parents. His brother Thomas and wife Bridget lived at No 78 Eliza but I believe his parents were at another house in the street (not confirmed). I cannot find a firm address for Frederick in 1917.
Although I initially could not confirm that Bridget (married to Thomas) was a part of the family from County Sligo, since I wrote to you I have come across some more info. Bridget from the Sligo family (father Michael & mother Bridget nee Monaghan) had a brother called Charles. Charles had daughters called Sarah (born 1892) and Teresa. I had considered whether this could have been the Sarah who married Frederick, as closer to his age, but I have now discovered she went to the USA in 1910. Further, Bridget (who married Thomas) had a sister called Josephine. This morning, I located a document showing that Josephine went to the USA in 1912. On the passenger list, she gave her address as 78 Eliza Street!! So, this confirms that the Bridget who married Thomas was from the County Sligo family. This Bridget had a sister called Sarah who was living with her and Thomas (Frederick's brother) in 1911 and a sister called Josephine who also was living at that address in 1912. All these sisters, Bridget, Sarah and Josephine had a brother called Charles who had a daughter called Teresa, who was living with Bridget and Sarah in 1901 and Thomas and Bridget in 1911.
The only hiccups are (1) that marriage record of Frederick and Sara which gives her parents as Michael and ELIZABETH, nee Monaghan as we know that Michael's wife and the mother of the girls was in fact BRIDGET, nee Monaghan. I am guessing this must just be a mistake and (2) on Thomas and Bridget's marriage record, Bridget's father is listed as a mechanic. This is inconsisent with all other records which show he was a stonecutter/mason and had been a building foreman on the Letterkenny Cathedral.
I hope I have not confused you with all the detail. I find it difficult at times keeping things straight but I expect it will be even harder for you, not knowing the various characters.
Let me know if you have any thoughts but, on the face of things, I believe it is a fair bet that Frederick married Bridget's sister Sarah, even if she was 10 years older than him.
Many thanks for your assistance,
David