Mary .......... the line you mention is certainly included in The American Stranger which is known by other titles including I'm a stranger in this country

In the middle of the ocean shall spring a myrtle tree
If I ever prove unfaithful to the girl who goes with me

Indeed, there are so many variations on this song it really deserves a thread of it's own. It would be very interesting to discover if it is connected with "Some tyrant"