A fada (like a French acute accent, but a longer stroke over the letter - though in printing, the acute is used) is used *when the name is expressed in Irish*:

Ó Miacháin

But not when it's expressed in English:

O'Meehan

when only the apostrophe is used. They're never used together.

There are two forms of 'Owen' in Irish: Eoin, which is equivalent to John, and Eoghan, which derives from a word for a yew tree.

The Eoghanachta are those southerners.