Hi, I'm the Kevin whom Roger mentioned above. One of my grandmothers grew up in Killasser, actually just a couple of townlands away from Rubble. I don't have the answers to your specific questions, but I do have some other potentially useful or interesting info.
First off, you may already have seen this, but here's a link to more info about Rubble, along with an interactive map which you can use to look around the area:
https://www.townlands.ie/mayo/gallen/killasser/cuildoo/rubble/
At that page, there are further links to other info, such as Griffith's Valuation and the 1901 and 1911 censuses. As Roger mentioned, Richard Kennedy is shown as a tenant in Griffith's, and I agree with Roger that he is probably who you think he is, in part because the Kennedy name is very rare in the area (most people in Rubble were Durkan's, and I have Durkan's in my family). I also noticed that there was a Sarah Rochford in the 1901 census, aged 55. She looks to be a good bit younger than your Sarah would have been, but I wouldn't place too much emphasis on that. In those days, few people knew exactly when they were born and there were often no records to check (I've seen large discrepancies of that type in my family). There was also a younger Rochford family: Michael (32), wife Catherine (25), and two children: Mary (4) and Catherine (1). There were no Kennedy's listed.
Another thing to consider when searching in the early 1800’s is that the process of building Catholic churches was still underway, and not all rural parishes had their own church building. Where that was the case, people were sometimes baptized or married in a neighboring parish. The main church in Killasser dates from the 1820's, and the "half-parish" church in Callow townland (where my grandmother grew up) was actually founded as a chapel even earlier, but things were difficult back then and not many records survive from earlier than the 1840's. The records for the neighboring parish of Swineford/Kilconduff start much earlier, and I know for a fact that some of my Killasser relatives were baptized or married there,especially if it was closer. Rubble is close to the parish boundary, so you may want to look at that parish register:
https://registers.nli.ie/parishes/0133
What you might also enjoy is a book that was written not too long ago about the history of Killasser parish: "Killasser: Heritage of a Mayo Parish", by Bernard O'Hara. It is sometimes available on Amazon. I glanced briefly at my copy, and didn't find anything about any Kennedy or Rochford families, but it doesn't have a comprehensive index.
Finally, there's another source which might interest you. There is a web site in Ireland which can be helpful to people who have identified a particular parish as relevant to their family history. In the 1930’s, Irish schoolchildren were sent out on class projects where they interviewed mostly older people in their parish about the history and customs of the area. Some of the interviews were conducted in English and some in Irish. Over 250,000 interviews were conducted and they are available online at the Irish National Folklore Collection Project’s site:
https://www.duchas.ie/en
The content of the interviews varied from place to place, but they can cover local history, local place names, people known to the person being interviewed, local myths and superstitions, and other customs, such as how holidays or weddings were celebrated and even on what day of the week people customarily did their washing or baked bread. It’s said to be the largest national folklore collection in the world, and the interviews can be fascinating. There are 36 interviews from Killasser which you can see (and download, if desired) here:
https://www.duchas.ie/en/src?q=Killasser&t=CbesTranscript
