I took your advice and visited the "Ancestry Ireland" website and signed up for a membership. On the "History from Headstones" website it states that if you have a membership on Ancestry Ireland you can also access their information. I purchased credits; however am still not able to view the items that match my search criteria. I've emailed the website owners to see if I am doing something wrong.
We have also been in contact with Rev. Gary at Dumcree Parish. He has asked the curate to do a search of the graveyard for us for possible relatives. This was so nice of him! We are still waiting to hear back.
I also looked at the "Title Applotement" site you mentioned and found a record for "Isaac" in Errigal Keerogue. I'm not sure if this is our "Isaac" as the 3 brothers came to Canada during the 1820's and 1830's. We are still searching for any relations (possibly from the brother Abraham) that are still in Sixmilecross.
I believe if we could just find a marriage record in Ireland for one of the brothers this would help us to find confirmation of who their mother was. We are hoping this answer may come from the Dumcree Parish graveyard when we hear back from them.
Unfortunately, the 'Robinsons' you noted in your response don't seem to be our line. But thank you for sending the link!