I have Walsh, Flanagan, and McAndrew relatives from Mayo, though mine lived/live a few parishes to the east of yours, in Killasser, Swinford, and Killedan (I've turned up some DNA matches in the Crossmolina area, though, so we are possibly distant relatives). I thought you might like some info about the families:
The surname Walsh/Walshe is an anglicization of the irish Breatnach, meaning Welshman, and is now found throughout Ireland, but there is a sizable pocket of them in eastern Mayo. I have a Walsh second cousin who still owns the house in Killedan where my grandfather (her great-uncle) grew up.
The surname Ó Flannagáin (from the Irish word flann, meaning ruddy) originated separately in several parts of Ireland, but the largest sept (or tribe) was in Connacht, and is believed to have orginated in eastern Mayo or just across the border in Roscommon. A couple of years ago, I located a Flanagan second cousin here in the US through my DNA results.
In Irish, the surname McAndrew is Mac Aindriu. According to MacLysaght’s Surnames of Ireland, the McAndrew surname was adopted by a branch of the Barrett family in Mayo. The Barrett’s were an Anglo-Norman family which became completely Hibernicized, and their surname became Bairéid in Mayo and Baróid further south in Munster. There is also an untelated McAndrew "clan" in Scotland.
My great-great-granddmother, Bridget McAndrew, lived in Killasser, and I've located a number of McAndrew relatives there. In the various branches of my family, people often married spouses who lived several parishes away. I thought I would mention that, because, although your Owen McAndrew married in Crossmolina, he might have grown up elsewhere (weddings usually took place in the bride's parish).
I don't know of an Owen McAndrew among my relatives at that time, but there is a lot I don't know about that branch, and the name Owen (Eoghan in Irish) is found in my family. My grandmother said that young peiople from all the parishes used to go dancing in Westport, where they met one another. That was at the turn of the century (early 1900's), but she said that they also met at fairs or at the weddings of relatives, and that could have happened in earlier years, too.
You may also be interested to know that that part of Mayo was still largely Irish-speaking when your family lived there, in some places even into the early 20th century. Both of my mother's parents grew up speaking Irish at home.
Aso, did you notice the listing from July of 2018 by someone else looking for Walsh's from the Crossmolina area? You might be related, and so might want to speak with that person.