The ironic thing about Buck's confession to Huck as to how the long-standing feud between the Grangefords and the Shepersons began is that he does not even know how it all started.
This irony is also a very funny part of the drama in chapter 18:
"What was the trouble about, Buck? -- land?"
"I reckon maybe -- I don't know."
"Well, who done the shooting? Was it a Granger-
ford or a Shepherdson?"
"Laws, how do I know? It was so long ago."
Buck was very good at explaining to Huckleberry how the two families have been at each other's throats for generations. He explains with great pride to Huck that a feud of this nature involves murdering each other's family members until everyone basically kills each other off...and then the feud is over.
Huck was curious to know what was the deal between the two families because their feud was a particularly bloddy and fresh one. The families would even carry their guns to the church services that they attended in order to keep an eye on each other. What we see in the behaviors of the two families is that they portray the ridiculous views on family honor that many families had for many generations in the smaller Southern towns of the US where one family wanted to keep control over the village, and another family would challenge them.
However, in an ironic and comical twist, Twain shows the ignorance of both families by noting how Buck, a member of the younger and most recent generations, does not even have a clue about how it all began. This basically means that Buck is fighting for a cause that is completely unknown to him and that the only reason why he is involved in it is because the rest of his family, and their ancestors, have all been involved in it- perhaps they didn't even know why either!