What I can tell you on my McDonalds/McDonnells is this: my great-grandfather, Richard McDonald, was born 1863 in Bolton. His father Patrick, according to the 1871 census, was born in Mayo. (Patrick's parents were Richard and Catherine). As Patrick was only about 30 years old in 1871, I think he must have come to Bolton much later than your people. My McDonalds lived at various addresses in Spring Gardens. I haven't found a lot of McDonalds in Bolton, however, so I would suspect that both families being from Mayo, there may be a connection.
Patrick came to the US around 1872, to New York. I know his wife's name was Catherine Prendergast (various spellings). I think I've found them on a passenger list (Patrick, Richard and another son John), but no mention of Patrick's wife. Patrick is my brick wall...just to finally discover they were from Mayo was huge!