Mc Donald or Mcdonnell of Roscommon

I am researching the name Percy Patrick Mc Donald born Roscommon in 1886.  On the 1911 UK census it says he was born in what looks like Roscommon in the county of Roscommon.  His name seems to have been confused some of hs children were called McDonnell and some Mc Donald.  He was my Grandfather and my father's name was Mc Donald which I seem to believe according to my mother was the correct one.  His father's name was Martin who was a farmer according to his marriage cert. he lived in Norwich when he moved to UK and was in the Royal Hussar's in 1907.  He was never registered in Ireland on the BMD I have had a search done.  Does anyone know anything.  One other thing I was told by someone on the Roscommon site that possibly his name was originally Patrick and he called himself Percy.  Joyce