Cassidy claims that the expression boyo can be explained in terms of the Irish language tendency to place an ó after a vocative expression, as in a mhic ó (oh my son) or a rún ó (oh my secret), though Cassidy failed to understand that this is associated with vocative uses. He gives the example boc ó (which he regards as the origin of bucko) and which is not a vocative expression.
In reality, boyo is common enough in Irish English but it is not “a bilingual appellation and expression of affection”. It’s an English word with an o stuck on the end of it, like thicko.
Incidentally, boyo is generally regarded as more typical of the English of Wales than of Ireland.