Further info: 1. I made a list of John Reilly and Mary Macken's entries that I found. I will attach it here and hope it's readable.
2. Important: My Grandfather, James Reilly in Glasgow with these parent names: I thought that he was the John Reilly twin shown on the list but using the name James. New information has come to light that James is indeed James and his brother John is found as staying at James' address in Glasgow when he married! So now, I don't understand why there is no James showing in Ireland to these parents. Both James and John in Glasgow, show the same parents names. On 18 Jul 1898 James marries in Glasgow and his age shown as 24. On 1st Jan, 1907 John marries in Glasgow and his age shown as 28, so born c1878? Since no record of a James, perhaps Patrick born 1874 was named Patrick James ... or should I look elsewhere other than Co. Cavan to find another John Reilly and Mary Macken as parents? I am not sure how to proceed now.