i have to say that i have never run into traditionalist irish who like the bouzouki and won't allow guitar. my experience is some won't allow any fretted stringed instruments. they do not appear in older irish music, however they have a big part in the revival. the bouzouki does have a celtic provanance of sorts by the way. scotts played the english guitar and other citterns right back to the early renaisance. the bouzouki was a cheap and available instrument that filled that role in the revival along with the portuguese guitar. you'll find as bands got richer the guitarras and bouzoukia were often put aside for luthier made modern citterns to fill that roll. in ireland there were no such instruments of any number but they picked uo the sound, especially after sweeneys men tried bringing the mandolin sound of bluegrass into modern irish.   a return of the american scottish irish rooted tradition back to the homeland. people loved it and hence the double strung fretted instruments took off like crazy.   
however traditionalists were not amused and still will gold sessiuns where you can't play a fretted instrument. me i'n cape breton gaelic, so i don't feel any problem with adding fretted instruments including the guitar. in the trip over the water we lost all the traditional instruments except the fiddle and bagpipes. the early clearances and homesteading was rough so they were mostly illiterate and played little music that couldn't be kept in your head. so i a way the traditionalist singers were un accompanied but only by force, as son as they got the werewith all to get more instrument they played them, guitar and piano were common as they were easily available and complimented the music.

the idea that accapella is more traditional is neo traditionalists trying to make a virtue out of necessity. the musically challenged can't play very well so they say no one did. the real reason they played solo was poverty plain and simple in england and the celtic countries. most people started playing instruments as soon as they could get them. that same neo traditionalist thing led to wierd notions. childe once visited the west coast of ireland in the midst of the gealic revival and said "i found much filigree and orniment but little that you could call music" pure class and ethnic bigotry. he wanted his folk to fit a preconcieved idea and he didn't collect songs that didn't fit it. such as songs of working class rebellion and struggle and songs that were bawdy and i assure you the clts were never as parsimoniuos as the english. in canada helen creighten was similar. she ignored songs of poverty and class revolt in english. she couldn't speak gaelic or french so when she collected the franco-gaelic songs she included the bawdy and revolutionary as she didn't know it was there and her early translators didn't inform her they just collected them separately or didn't translate them. she was trying the find a folk music that reflected her south halifax bourgeoise world view.

those atacking the guitar as non traditional are doing the same thing