The word ballyhoo was originally carnival slang for the spiel which a showman used to get people to gather round his show. It is first recorded in this sense in the early twentieth century and around the same time, it is found in the more general sense of a ruckus or a hubbub.
Now, Cassidy (and Loretto Todd before him – one of the rare examples of a word which both of them claimed for Irish) claimed that this word comes from the Irish bailiú, which means gathering or collecting. There is nothing terribly improbable about this claim but neither is it a great fit. A ballyhoo is a loud thing full of razzamattazz. Bailiú is rattling a box for St Vincent de Paul outside of mass on a Sunday.
And then, of course, there is the little matter of alternative claims.
It seems that the most likely origin of the word is a shortened form of Ballyhooly, which meant a ruckus or rumpus or fuirse-má-rabhdaileam (what a great Irish expression that is!) This apparently derives from Ballyhooly in Count Cork. According to the OED, this came about because Ballyhooly was famous for faction fighting. While one irate local has commented on line that this is a slur on the good name of Ballyhooly, the idea that Ballyhooly was a byword for faction fighting is confirmed in a book called Cork Past and Present, published in 1905:
“At the close of the 18th century, there was a poor and plain Catholic Church about a mile from Ballyhooly but on the southern side of the river Blackwater. It was built on the side of a hill on the road to Rathcormac.
In the year 1819 a lamentable scene was witnessed within its walls. A few of the parishioners were at enmity with some of their neighbours and even in their place of worship and heedless of the counsel of their priest, they could not restrain their angry feelings, but came to blows.”
Apparently at least one person was killed when the riot spilled out onto the roadway outside. By the 1830s, to ‘give someone Ballyhooly’ meant to give someone hell, as in an 1837 story set in the west of Ireland in Tait’s Edinburgh Magazine.
In other words, a shortening of Ballyhooly is a pretty good fit for the origin of ballyhoo – at least as good as bailiú, anyway.