There aren't many "Round Churches" in the world, the wikipedia page of Round Churches has only listings of a little over a dozen but I'm not sure of course if this list is complete, I assume it is not. What can be learned though is that round churches are often found in Sweden and Denmark and were popular church constructions in the 11th and 12th centuries.
The Round Church in Bowmore, belonging to the Parish of Killarrow, was built in 1767 by Daniel Campbell of Shawfield and Islay. Walter Campbell, who succeeded Daniel and continued to build on what he started, died in 1816 and is now commemorated by a white marble obelisk headed mural monument, see picture below.
Now why was a round church built in Bowmore and not a rectangular or square one? The story goes that the church was built in a circular shape to keep the devil from hiding in a corner but funny enough this is not supported in any of the documents to be found online. Perhaps Daniel Campbell, who had the church built at his own expense, just preferred a circular shape? Whatever the reason is, The Round Church is the only one of it's kind in Scotland built in this period.
Rounch Church Interior
Round Church Interior
The white marble monument for Walter Campbell