Well, Les, the plain-and-simple answer is that the Catholic Church requires religious songs for religious services, and "Danny Boy" is not a religious song (notwithstanding the fact that it does say that the singer will pray for Danny after he's dead). It's a nice, old, sentimental song that a lot of people like, but it has nothing to do with liturgical worship. However, most Catholic parishes allow it to be sung before Mass, or after the main part of the Mass is over.
The general idea of a Catholic funeral is that it is supposed to be a joyful celebration of a coming home to the Lord after death, recognizing and consoling the grief and loss of those left behind but conveying a message of hope and faith in a life hereafter. "Danny Boy" is a nice song, but it doesn't convey that message.
This is a change from the mournful, sorrowful, fearful tone of the "requiem" Mass of the past, which spoke in medieval terms of "that day of wrath, that dreadful day." The new approach is more consistent with a traditional Catholic faith and theology that pre-dates the somber tones of medieval and Victorian times.
Sorry you have such a cynical view of the situation, Les er, Greg. I think I'd agree with those who say that an extra hour of testimonials as part of an already hour-long funeral Mass is too much. Most parishes I've seen try to keep it down to ten or fifteen minutes at the end of Mass, and then allow for more at the reception in the church hall afterwards - or at the graveside. That allows an opportunity for a gracious exit for those who can't stay longer. Yes, I've known liturgists and priests whose primary concern is to get the damn thing over and get out on the golf links - but my experience is that most of them try their best to do a good job with good taste and with compassion for the people involved. I was in a Catholic seminary for 8 years and majored in Theology, and I've served as a catechist and church musician for over 40 years, so I have a fair amount of experience - most of it good.
-Joe Offer-