According to the Irish Times of 11 February 2017
Margaret Gaffney was born on Christmas Day 1813, in Tully, Co Leitrim. Five years later, faced with extreme poverty and religious persecution, her parents and the three youngest of their six children, including Margaret, boarded a steamer bound for Boston.
Eoin Butler, the author of the article, provides no details of the vessel, but I hope he will: up to now folk have believed that an American vessel called the Savannah was the first to use steam on any part of the Atlantic crossing, and that was in 1819, the year after Margaret Gaffney’s crossing.
Irish waterways history 
Margaret Gaffney could have boarded the Sirius on 4 April 1838.
Sirius claimed to be the first trans Atlantic steamer,
From Waterford Chronicle – Saturday 26 May 1838
Extract of a letter from Lieutenant Roberts. Sirius, off Falmouth, Saturday, I9th May, 1838.
We started from New York 1st of May, fine weather; two days after we encountered heavy gale from S. E., which lasted five days ; in fact we had only three days fair wind the whole passage home. It was very fortunate I took one hundred tons of coal more than I first intended; but the first trip thought it would not do to be out of coals. I purpose running into Falmouth to take in coals and start at once for London. The Great Western was leave on the 7th. had eighteen days passage out—head wind all but five days—some very heavy gales. The Sirius is noble vessel. She never can be rivaled as a sea boat. We have gained all that we want, at least I have, that is. to be the first to cross the Atlantic to and fro. The Great Western arrived the day after us.
So (at least) two steamers crossed the Atlantic in 1838
Indeed, but the Irish Times claim is that she crossed in 1818, twenty years earlier. And, IIRC, neither the Sirius nor the Great Western spent six months on the crossing and neither of them went to Baltimore. Other dates given in the story show her in America years before the Sirius got there. bjg
apologies, I was confused
Aren’t we all? No problem! bjg
Three other points about the Irish Times story:
(a) people in “extreme poverty” did not engage in transatlantic travel
(b) I doubt if the passengers would have brought food, even starvation rations, for even half the six months the crossing is said to have taken
(c) the “steamer” bit also appears in the Wikipedia entry for Margaret Haughery.
bjg