Usually the song 'Danny Boy' is there to heighten or foreshadow tragedy. Leo plays it while lounging around in his bathrobe when guys burst in with tommyguns in hand and their hats on their heads, he sure looks like the Sacrificial Lion. But not quite.
For added badass points, Leo casually relights his cigar with his smoking tommygun while gazing at the flaming wreckage of the escape car, along with a little nod as if to say "Nice try."
The sequence in the forest where Tom is ordered to execute Bernie. With Bernie - with an incredible over-the-top performance by Turturro - literally begging for his life. "Look into ya heeaaarrtt..."
And the response that takes the rest of the movie to get to. "What heart?"
The Dane calmly threatening Verna in an empty boarding room, casually leaning to avoid a low-hanging lamp (he's that tall) as he stalks her.
How Tom is able to - while getting the crap beat out of him by an enraged Dane - convince Caspar that his loyal Dragon is the one aiding the devious Bernie.
Tom is probably one of the greatest examples of a protagonist Magnificent Bastard ever. With a little luck, he manages to destroy the Italian mob using a Zero-Approval Gambit and downright brilliant manipulation tactics to see it tear itself apart and ensure his boss comes out on top.