The two elves cross the lake first. Elf#1 rows back.
Leprechaun #1 crosses. Elf#2 rows back.
Two elves cross. Elf#1 rows back.
Leprechaun#2 crosses. Elf#2 rows back.
Finally, the two elves cross.
Minus' favourite feeder, John Moser (firstname.lastname@example.org), provided yet another tantalizing mouthful for him to feast on:
The two elves go over together. Elf B stays on the other side. Elf A returns to the original side. One leprechan goes across by himself. The elf B returns to get elf A. They return to other sdie again.
For variety this time, elf A stays and elf B returns to the original side. The second leprechan goes across by himself. Elf A returns to pick up elf B. If they went quickly and the orge was slow enough, elf A should be able to bring elf B back to the safe side of the lake.
Edward Chang (email@example.com) had no problem finding the solution to our problem:
First send 2 elves to the other side and then one of the elves come back and one of the leprachaun takes the ferrie to the other side. Then the elve that went over goes back and then both elves go across the river and one comes back, Now the leprachaun goes over and the elve drives the ferry back and both of them goes across the river.
Bob Shapiro (firstname.lastname@example.org) gave Minus this solution. Although it is not a very complete answer, we have to give him credit for adding a touch of humour...
The two elves could go on the first crossing, then the ferry could return for the leprechauns who would have to cross one at a time. It would take a total of three trips for them all to get across, by which time the ogre would probably have eaten several of them.