Hi Mary, how wonderful that you will be visiting County Mayo later this year!
Have you tried www.rootsireland.ie - it has transcriptions of both church and civil records of births, marriages and deaths and I was able to find birth and baptismal records for your grandfather and his siblings (including some who appear to have died young) as well as the marriages of his parents and his maternal grandparents. I was also able to locate the birth record of his mother Mary as well as the birth records of her siblings.
Once you have this information from www.rootsireland.ie, you will be able to obtain the birth and marriage records for free on the website www.irishgenealogy.ie.
If you haven't already, I would also recommend you look at the 1901 and 1911 census returns which are available for free on www.census.nationalarchives.ie where you will be able to find the households of your great-grandparents as well as Mary's parents; both in Killala.
Using the names of Mary's siblings, you could then search www.rootsireland.ie for their marriages and perhaps even baptisms of their children.
You may wish to contact the North Mayo genealogy centre in Ballina which is responsible for the Mayo records on www.rootsireland.ie - they offer various research services for your forthcoming trip. Their email is
northmayo@gmail.com.
All the best with your research and have a wonderful time in Ireland!
Gillian
Gillian Hunt
Research Officer
Ulster Historical Foundation