Susan's advice covers the question so well that it ought to be a business plan--

particularly important the point that your friends really want to play for receptions, not the wedding per se(even though people will call them "The Wedding Band")--And her point about lead time is also really important--keep in mind that the most formal weddings(which are the ones where they pay good money for musicians) are being planned a year or a year and a half in advance--So if your friends decide to do this, they should plan to be in it for the long haul--

Another opportunity to be aware of--Wedding Planners often offer "Showcase" programs for their prospects--this is a day long event where all the suppliers involved show their wares, including musicians--hard to get into, but when you do, you'll get a lot of work--

Also, this is a highly competitive market, and they will be expected to be polished and professional--they must dress appropriately, they must have an adequate sound system for each facility--they must co-ordinate their music with the all the activities that will take place during the reception, they must make announcements and introductions when necessary, and they must play whatever song the bride and groom choose for their first dance, even if it isn't Celtish--and, especially, they must be prepared to play the whole raft of corny Irish and ethnic songs and dances
that people expect at weddings(Hava Nagila, the Tarentella, Danny Boy, Danny Boy, and Danny Boy)--

Also, the band should be prepared to be totally ignored if things are going well, and to be in the middle of things when anything goes wrong(even when they have nothing to do with the music)

If your friends say, "Oh, no, we want to just play this, this and this" remind them of the old axiom that he who pays the piper calls the tune--Also, that the reason that the money is so good is that you have to work real hard--