On the surface this is a well-written story about an American woman coming back to the family house/farm in Ireland and upsetting the status quo. Alice Lennon had left Ireland for a bad marriage and lived an unsatisfactory life in America for a couple of decades. Meanwhile the Lennon farm ("Watergate") had been taken over by her weak/feeble sister and her equally weak in spirit brother-in-law, Martin. Her other brother, John, lives nearby and watches the decay of the family farm in dismay, but doesn't do anything. (Yes, his name is "John Lennon"...that made some odd reading moments.)
Through Martin's charity, a tinker woman (probably gypsy/Roma) and her touched daughter have ensconced themselves at Martin's farm, ostensibly as housekeepers, but really as malingerers and takers. The house is decaying and the farm is not producing - obviously this can't go on. And then Alice comes to stay and her energy totally upsets everything as she quickly susses the situation.
Now this is all good house-drama stuff, and I could even see a play come out of the dramatic possibilities. But there is something else going on here that makes this book a little more than a family squabble in rural Ireland. This book was published in 1942, so it was probably written in the 1940-41 time frame. What was going in Europe then? A hell of a lot! The Germans have taken advantage of the appeasement of the English, French and other Europeans to take over Poland, Czechoslovakia and soon to be France. What is the risk of doing nothing? What is the risk of doing something? Bad choices either way, but it is the fact that the Germans aren't going to leave on their own that creates the situation.
I see direct parallels with the European situation at the time with what is going in "Watergate". Obviously the Germans are symbolized by the "foreigner" tinker (Ruby Butts - what a name!) taking over the Lennon farm. She even has a previous conflagration in her past (like WW1 for example). The Lennon's that did nothing are the appeasing European countries, and Alice Lennon (the American) is the United States having to come in and help clean up the mess.
I don't know anything about MacManus and his views on the English, but the name "Lennon" is not obviously Gaelic to my ears. There may be some local Irish politics going on here as well with Watergate being a symbol of English colonialism being corrupt by nature and the Lennon's got what was coming to them.
As far as the characters in the book - they are not at all two-dimensional as it may appear from the above. There are complex motivations and even Ruby Butts does have some sympathy from the reader, though it seems MacManus may have some prejudice on the proclivities of the Irish travelers.
The only structural problem I had with the book is that the narration was unclear. Ostensibly it was by a farm-hand on the other Lennon farm (Peter) who has literary ambitions, but he doesn't have a role in the book other than witness. Sometimes the narration seems omniscient and then sometimes personal, leaving a "who is saying this" feeling to the reader.