For "Old Ironside" if you have to choose just one writing technique, I would focus on either imagery or verbal irony because Holmes relies heavily on both devices. For imagery, consider all the examples of images and sounds present in the poem, from the "tattered" flag to the red deck to Holmes' comparison of the ship's cannon to a meteor. The poet's decision to use imagery helped ignite readers' emotions toward rallying for the ship's preservation.
If you choose to focus on verbal irony, discuss Holmes's tone (one of bitter irony) demonstrated by lines such as "Ay, tear her tatter ensign down!" (Line 1) or "Oh, better that her shatter hulk / Should sink beneath the wave" (Line 17). The verbal irony causes readers to consider what kind of nation would "thorw away" something as valuable as the USS Constitution.