Complex sentence: Robert Emmet, mentioned in the poem, planned for a revolution unsuccessfully several times, while, when he was finally successful he was said to try and stop everything mid-rebellion because he witnessed a man being pulled from his horse and killed.
Split into simpler sentences: Robert Emmet, mentioned in the poem, planned for a revolution several times, unsuccessfully. When he was finally successful, he was said to try and stop everything mid-rebellion, because he witnessed a man being pulled from his horse and killed.