Can you point out any ironies in the folling episodes: (a) the Grangerfords and the Shepherdsons in chuch, (b) the sermon, (c) the elopement of Harney and Sophia?

The scene in church is ironic. You have these two very well-to-do families, all dressed up in their Sunday best, going to church........... all of them packing guns and ready for a fight.

"Next Sunday we all went to church, about three mile, everybody a-horseback. The men took their guns along, so did Buck, and kept them between their knees or stood them handy against the wall. The Shepherdsons done the same. It was pretty ornery preaching--all about brotherly love, and such-like tiresomeness; but everybody said it was a good sermon, and they all talked it over going home, and had such a powerful lot to say about faith and good works and free grace and preforeordestination, and I don't know what all, that it did seem to me to be one of the roughest Sundays I had run across yet." (Chapter 18)

The quote above also pertains to the irony of the sermon. You have a room full of guns and a sermon on brotherly love.......... the preacher- he had nerve :-D

The elopement of Harney and Sophia is your typical love between the children of feuding families. Her treatment of Huck is the irony, in that she flirts, he falls for it, and he believes her to have a certain fondness for him........... then he finds out she's run off with another man!