"Shamus (or Seamus) O'Brien" was a standard old-time waltz for many Ontario fiddlers in the 20th century. The melody comes from an 1867 Irish-American music hall song by William Shakespeare Hays - "Sheamus O'Brien; Answer to Nora O'Neal". There are a million different ways to play this tune. This is more or less how I do it. The minor key B part is wonderfully melodramatic. I've found it's good for accompanying the waltz-clog stepdance. Reg Hill plays his version in the YouTube link below.
Still from a 1912 movie of "Shamus O'Brien" - based on a popular poem from the 1850s