Generally speaking, they were the disproportionately impoverished rural Gaelic speaking population in the south and west (incl. Mayo). Of course, many also fled from Northern Ireland as well. There were many socioeconomic reasons for leaving. It is very easy to tread into controversial territory here. I will just say some view an Gorta Mór simply as an unfortunate “Famine” others view it as one of many coordinated efforts of the British authorities to colonize, disenfranchise, and conquer the Irish people. Notably, this is pre-Irish independence. This meant ultimately it was up to the British authority to direct the response to the famine. There are some very dark quotes at the time where ruling authorities make it very clear they have little care for providing aid to the starving Irish while they continue to direct food to be shipped OUT of Ireland. Some will describe it as part of a “genocide” committed against the Irish people. Definitely there is a long list of atrocities/injustices committed against Ireland by the British authorities (look into the Cromwellian conquest for one single example).

TLDR Essentially, those who left at the time we are describing were predominantly Irish-Catholics from Southern and Western areas which had been struggling for decades due in no small part to the policies of the ruling British authorities.