On the pig's back
To be in luck; in a prosperous happy state.
If ever a phrase were unequivocally Irish, it is this one. 'On the pig's back' is a literal translation of the Irish 'ar mhuin na muice', which is used colloquially to mean 'well off'; 'in luck'. That Irish version is known from the 17th century onward and the English translation of it has been in use since the 19th century; for example, this piece from the New Zealand newspaper The West Coast Times. August, 1890:
If one third of the yarns about the size and quantity of the mica deposit in the district have any foundation in fact the Greymouth syndicates are on the pig's back.
.. and back in Ireland, in John Whitbread's play Lord Edward, 1894:
"Begorra he's on the pig's back this time."
The notion of riding on a pig's back leads inevitably to 'piggyback'. This term, in its original form at least, predates 'on the pig's back' and also the apparently related phrase 'high on the hog' - and all these expressions have some association with enjoyment. Nevertheless, I can find no connection between them. Many English expressions and proverbs relate to pigs, no doubt because, as a race, the English have had much close contact with them over the centuries. The similarity of these three phrases appears to be no more than coincidental.