The Wikipedia entry (at least the English one) only addresses the origin of the English word, and even then presents folk explanations by food writers. If the first English reference in 1430 predates the Gaelic, then the Gaelic word may come from the English. Clearly, going from h- or a vowel to t- can't be a Gaelic vocative. Could the t- be a consonant that wasn't originally part of the word? If the h- were not pronounced, would the definite form have been something like an t-haggis?