You could have a look at the more recent death records for that area on http://www.irishgenealogy.ie. They go up to 1965. Ballinsloe would be the relevant registration district. There’s 12 Mitchell deaths in that SRD between 1950 and 1965. Marriages on the same site also go up to 1940. You can use the commercial sites to search indexes only for marriages up to 1958.