I'm not sure if you're indicating that you don't know where Ballybeg or Buttevant is located. From what I have found on a very old map (attached), Ballybeg is just south of Buttevant. There is an old priory located there along the banks of the Awbeg river.
Based on the marriage date of 1848 and assuming that William could have been about 20-25 years old at the time (big assumption), he could well have been a sibling or close cousin of Edward Linehan (my 2x g-grandfather). When I had researched baptism records via rootsireland.ie I don't recall coming across a child of Cornelius\Mary Linehan with the name William. It may very well exist, I just didn't find it. These are the siblings that I "think" are the family of Cornelius\Mary(Kearney) Linehan:
Michael (1818-1899) Emigrated to New York\USA 1844
Edward(1833-1913) Emigrated to NY/USA Nov 1851
I indicate "think" because the only evidence that I have for the origin of this family is a letter written by Cornelius (in 1867) vouching for Michael as being born in Grange\Buttevant in 1818 and a death cert for Michael that lists Cornelius Linehan as his father and Mary as his mother (but no maiden name). I found these records via rootsireland.ie and these were the only records I found which contained the parents names, town of Grange\Buttevant, and expected childs name (Edward).
I will also mention that there was an Edward Linehan who was a contemporary of Cornelius', also from Buttevant and who was married to Mary Egan. One of their children (Catherine Linehan, married Cornelius Relihan) also emigrated to NY\USA and lived very close to my gg-gf Edward and his family in New York (Edward was married to Cornelius Relihan's sister, Ellen).
The Relihan's were from the Churchtown\Ballinguile area, just north of Buttevant.
So, it's possible that William Linehan could have been a child of EdwardLinehan\Mary Egan.
Lots to ponder.