I'm not from Killasser but have ancestors from the area, so I did a little searching for you.
The Griffith Valuation in Mayo was prepared in 1856-57. I didn't find any Marron's in the parish of Killasser, but there are John Marn Sr. and John Marn Jr. in Carrowliam More. Could this possibly be yours?
http://www.failteromhat.com/griffiths/mayo/killasser.php
In 1901 you can search again for the family in the census. The Leitrim -Roscommon website has a search option for parish. If you type in Killasser in the parish field, you can look for variations on the name spelling. There are only 3 families with the last name Marren (no Morren found) in Killasser again in the townland of Carrowliam More: William (56), Michael (59) and Patrick (60). The three were therefore born between 1841 and 1845. One could possibly assume they are sons of one of the John's and siblings to the group that left for the US.
http://www.leitrim-roscommon.com/1901census/census.shtml
In 1911, you find all the families still in Carrowliam More: William's children John and Jane Marron, Michael's widow Bridget Marren and children, and Patrick Marron (79) and family.
http://www.census.nationalarchives.ie/pages/1911/Mayo/Tumgesh/Carrowlia…
If you look at the actual census record for Michael's widow Bridget, you'll notice that oldest son Patrick J. Marren was a National Teacher. I have the book Killasser, Heritage of a Mayo Parish by Bernard O'Hara. There is a chapter named Profiles of Some Killasser People that includes a profile on Patrick and his wife Eileen Marren. Patrick, native from Carrowliam More was a schoolmaster in Knock. There is a nice tribute and a picture of Patrick, his wife Eileen and son Fr. Vincent. There is also another profile on his other son Martin "Enda" Marren, who was a well known solicitor in Dublin. (You can google him.) There is a third profile of a Thomas Marren (1949- ) from Cloonfinish.
If you feel this is a viable connection, perhaps you could specifically search in these couple townlands for these families. Here is a cemetery link that includes several of the Marren's that might also be of help.
http://irishgraveyards.ie/search.php?yardid=0&lastname=marren&firstname…
Cathy