Interesting to note that while O'Reilly itself is Anglicised from the Irish surname Raghallach (it means a sociable people), most variants of Reilly (i.e. Ryley, Riley, Ryleah, Ryleigh, Reily etc) are often actually English and not Irish at all. They come from the old English "ryge leah" - a rye meadow. Various English villages/towns have variations of the name. And to confuse matters further there's an old Devon/Cornish Celtic name of Reily/Riley. Cornish is a bit like Gaelic, but in Southwest England, and the name developed separately there through that language.
It brings to mind another oddity, the surname styled as O'Dell. Yes, it looks Irish, and many who use it are in Ireland (and later the US etc) but its origin is in fact the entirely English surname Odell (specifically, Bedfordshire in England). But some Odells who emigrated to the Limerick area in Ireland during the British rule there, later added the apostrophe (and other folk who don't know just assume it's Irish and add one too).
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It brings to mind another oddity, the surname styled as O'Dell. Yes, it looks Irish, and many who use it are in Ireland (and later the US etc) but its origin is in fact the entirely English surname Odell (specifically, Bedfordshire in England). But some Odells who emigrated to the Limerick area in Ireland during the British rule there, later added the apostrophe (and other folk who don't know just assume it's Irish and add one too).