Well, I've got to agree that Halloween is a "holiday" with decidedly unholy origins, I'm guessing this "reverend" is either being sarcastic or has serious problems of the mental variety.
I mean, what does Mexican immigration have to do with the Irish potato famine? I assume he means Irish, and I have read elsewhere (from a more lucid author) that the large migration was responsible for the intorduction of this celebration to America, but I wouldn't exactly hold that against those immigrants who've contributed good things to this country.
Regarding the illegal status, I have no clue if that's the case (although it seems possible). If true, at least the children of illegal immigrants back then were committed to becoming Americans, not retaining their identity with their old country.
Apart from stating a bit about halloween, his piece is drivel.
Oh, and IBTZ
From Wikipedia.org:
"Halloween is most popular in Ireland, where it originated, also known in Irish Gaelic as "Oíche Shamhna" or "Samhain Night". The Celts celebrated Halloween as Samhain, "End of Summer," a pastoral and agricultural festival of fire, when the dead revisited the mortal world, and large communal bonfires would be lit to ward off evil spirits. (See Origin: Celtic observation of Samhain below.) In Ireland they continued to practice their deep-rooted, ancient pagan rites well after the arrival of Christianity in the middle of the sixth century."