Here's where I take issue with that ^^.
The original name, O Ceallaigh, was misspelled by the English as Kelly, or O'Kelly, or Kelley or O'Kelley. I myself have some 'O'Kelley' ancestors.
The letters K and Y are not part of the Irish alphabet, so to anyone who spoke Irish back in the day when language change was being imposed on Ireland, the name became unrecognisable and maybe even unpronounceable. The accent from the O was removed and the two separate parts, O and Ceallaigh, mashed together to form a nonsense spelling even from an English pov if the O' was retained, as an apostrophe has a certain role in English that it does not play in the name O'Kelly or O'Kelley.
So in actual fact it is from an Irish pov a case of the English taking a perfectly simple name (it obeys all Irish orthographic and pronunciation rules and Irish orthography is far simpler than English orthography) and turning it into something completely foreign.
Irish orthographic and pronunciation rules differ from English in many respects (because they are different languages, duh), not least of which is their simplicity.