By far the majority of the Irish who arrived in the US in the 19th Century spoke only English, or were bilingual in English and Gaelic.
The 1861 census of Ireland (a few years after the large potato famine immigration, but the best data I could find, and unlikely to have been much different in 1846) found that only 2% of the Irish population spoke only Gaelic. By far the majority spoke only English, and most that spoke Gaelic also spoke English.
As the Irish were one of the few immigrant groups arriving in the US in the 19th century already speaking English, this, of course gave them numerous advantages over immigrants who could not speak the language.