As an Irishman, I can confirm the information above.
Knock was probably not well known because there was no verbal message of Our Lady.
There is speculation as to the purpose of the apparition. One theory is that it had to do with la Salette. The Secret of la Salette was approved only 4 days prior to that apparition. A Heavenly approbation of the secret?
As for the modern monstrosity which they call a church, it was built sometime in the 70's/80's I think, obviously to conform with the spirit of Vatican II, with another pretext being to accommodate large numbers of pilgrims.
A few years ago they were taking out the Novus Ordo altar in it, and as they were doing so a large number of rats came crawling out. So reports go. Usually though, the rats in Knock are not of the tailed and furry kind. Nonetheless it is a great place to visit and the resistance organizes a pilgrimage there every year.