豪快 means "hearty" or "splendid," and because it is a na-adjective, adding に after it is like adding "-ly" in English. The イッちゃった is really a way of emphasizing the root of the verb いく, which (and I'm not joking here) is a slang for "to cum." Thus, It means "I came a lot," because いっちゃった is the past tense form of いっちゃう, which is いく+しまう which is "to cum" with an auxiliary verb carrying with it a feeling of regret or a sense of finality. In this case, it is the finality of having came, and being done with sex for that point in time.
"I CAAAAAAAME." is perhaps a more accurate way of translating it, then.