Clay,
David would have been 24 at the onset of the 1840's potato famine. I believe my own family emigrated at that time also.
There was only one port of emigration at that time and it was on the west coast. I believe it was the same city the Titanic sailed from.
You may be able to track him through emigration rosters from their records.
Although Butlers have immigrated to America since the 17th century, most of our ancestors arrived here as a result of the 1820's or the 1840"s famine.
Good luck.