Irish death certificates do not routinely record the deceased’s spouse’s name. Sometimes I have seen it say “wife of X” in the occupation column, and as you say, if the spouse was the informant, you sometimes get the information that way.
Irish death certs are not on-line anywhere (including the FMP site). They only have the civil indexes. You would need to order copies of the various certs from GRO Roscommon to find the information you want. http://www.groireland.ie/http://www.groireland.ie/