Can’t say for certain that it’s the right family but on the other hand there’s no sign in the Irish birth records of a Michael McGillon born to Michael and Ann around 1868.
Dates of birth in the 1800s need to be taken with a pinch of salt. Irish people then generally didn’t celebrate birthdays and often had no idea of precisely when they were born. If officialdom later asked for a date of birth, it was common to make one up. Likewise when parents registered a birth the date on the certificate was sometimes just a guess too, or an “adjusted” date to avoid a late registration fee. (Registering a birth was free if done within 3 weeks but after that there was a small penalty for late registration. Obvious thing therefore was simply to move the date to one that avoided the penalty).
Saying you were 21 when you married would make you an adult and might avoid any consent difficulties over actually being a minor. It’s possible he wasn’t 21 then. Who would know?
The Mc & O’ prefixes in Irish names are detachable and it was common to see them dropped, or re-attached, in the records according to whim. Most Irish search engines automatically look for both versions.
There are several trees on Ancestry that list Bernard Gillen and Ann Conway eg the Haffey-Danowitz tree. They list Michael b 1874 but have no other information on him save for his birth. Might be worth comparing DNA with someone on that tree.