First, the song utilizes layering of many different instruments in order to catch the listener's attention gradually. It begins with a lone fiddle, which is soon cut short by the bass line, which is present for only one measure and functions as a transition into the second part of the introduction. Then, the familiar fiddle returns, but with the addition of percussion and keys, which add to the "build-up" of the song. Then, the piano plays a glissando that transitions the song from the introduction to the first verse (and smoothes over a shift from the key of F major to the key of C major). Those instruments are generally used throughout the verse, until an accordion is added toward the end of the verse. The accordion signals the movement of the song from verse to chorus, and serves to change the key of the song once again from C major to D major. The song continues to follow this pattern of single instruments signaling transitions from section to section and from key to key.
As mentioned previously, Come On Eileen uses frequent key changes when transitioning to and from sections of the song. This distinguishes the tune from many commonly heard songs, and makes it interesting to the listener. The key changes can be unrecognizable to ears that are untrained in music theory, but anyone would be able to determine that there is a sort of difference between sections of the song. That indistinguishable difference can bring intrigue, mystery, and interest.
Come On Eileen is also distinguished by frequent shifts in tempo. The introduction and verse are characterized by a driving, intense tempo that undoubtedly pulls forward. This driving forward motion is emphasized by the consistent use of the heavy bass line. Then, the chorus continues with the quick, lively, tempo, but instead of driving forward, sits on that tempo and allows the listener a chance to "breathe" and just listen. This cycle between forward motion and stagnant bouyancy enhances the sense of euphoria created by the song, because a sense of tension is created within the listener and then rapidly released.
Suddenly, the tempo dramatically slows at the start of the bridge, after a ritardando at the end of the chorus. Throughout the bridge, it accelerates more and more, until the song seems to absolutely get away from itself. As the tempo accelerates, more layers of harmony are added to emphasize the tension-building effect. It finally culminates in two measures of rapid-fire solo fiddle that shift the song back into the chorus. The chorus returns to the same buoyancy as before, but with a faster tempo. This tempo continues into the end of the song and fades out with a diminuendo.
I never expected that I would enjoy applying principles of music theory to one of my favorite de-stressing songs, but it has proven to be fulfilling and interesting. I enjoy being able to pick out and analyze the concepts that make this song stress-relieving, energizing, and joyful for me, and I hope this song creates the same effect in other listeners.