Hello ... just read your positing to me. Fitzpatrick is a very common name in Fermanagh and Cavan, and Patrick is in almost every RC family in Ireland! So many Patrick Fitzpatrick's will turn up in your searches!
You say that on Patrick's birth reg there is a date and place underneath his mother's details which may possibly be the marriage. Do you have his mother's details here as well as this date?
Yes, Newtownbuttal could well mean Newtownbutler. If he got married in 1858 in Newtownbutler it may be difficult to find his marriage details. I have looked into researching the church records in Newtownbutler, and if I can, I am going to be going to search for my great-grandfather who is also Fitzpatrick and married in 1857 in Newtownbutler. I am not sure exactly when this will be as I have to travel there, and it could be later in this year before I get to do this, but when I am doing this I could have a look for Patrick Fitzpatrick and Mary Little's marriage in 1858.
In the information you already have, are there any details of either Mary or Patrick's parent's names? Or even the sponsors who attended their wedding? Or even the townland where they lived? If you have any details like this, it may help me to see if I can find out anything else for you.