Beads made from horn aren’t limited to Ireland, but they are surprisingly rare elsewhere. They are closely linked to the Mitchell Rosary Factory of Dublin; the earliest printed reference I’ve found to rosaries made of horn beads is from a report on handicrafts in Dublin, published in 1908. In this publication, the Mitchell family is mentioned as drawing on their family business making combs and other ornaments from horn, bringing this expertise to the making of horn rosary beads:
"There is one Irish policy about which everybody is agreed, namely, the development of Irish industries; and this is the policy of the Irish Art Companions. In 1904 they set to work, taking a house, a workshop, and show-room at Nos. 27 and 28 Clare Street, Dublin. Besides the crafts of modeling and casting, the Companions make rosary-beads and other articles from horn; and for this purpose they have allied with themselves Mr. Albert Mitchell, the son of the last of many Irish by-gone comb-makers." 
In his memoir ‘It’s a Long Way from Penny Apples’, Bill Cullen gives us a fascinating, and largely positive, look at Mitchell’s rosary factory in the 1950s:
“Alex Mitchell’s rosary-bead factory had hundred of girls making rosary beads to help Christians pray all over the world. There was a terrible smell from the factory when the girls were boiling the cows’ horns. That’s right. Made out of cows’ horns the beads were. The boiling softened the horns so the cutting machine could extract little round balls of horn. Then the spike machine stuck a hole through the middle of the ball. The balls were run through a dying machine. Different color every day. Some left natural. The drying machine hardened the beads. The polishing machine gave them a shiny gloss. The machines in rows worked by the girls, who wore white coveralls with a big loose white hat over their heads to keep the smell of the horns off their hair. Hundreds of colored shiny beads with a hole in the center poured into the collecting vat.
Buckets full of beads were given to the “decade girls,” who lined up at the workbench with their pliers and a roll of fine wire. Thread the wire through the bead, then cut the wire and make a little hook with your pliers. Attach another piece of wire and thread another bead. Cut and hook again. Until you had a string of ten beads. A decade of the Rosary. Drop the decade into your finishing tray and start again. You were paid by the decade… 
In the finishing room, other workers linked the decades together and added the crosses and centers. It appears that all of the factory workers were women, and this hang tag from 1947 seems to agree:
The work was dull, and hurt the workers hands. But in inner-city Dublin any work was a blessing. Mr. Cullen’s description of Alex Mitchell the elder is fond and respectful.
“The work was boring. So the girls gossiped… And they sang. Every song ever written. Taking it in turns, with everyone joining in the chorus. Old man Mitchell was a gentleman. Always smiling. Always stopped to have a word with the girls. Knew most of them. Gave a few pounds’ bonus at Christmas.” 
Not everyone held as positive a view.
"And what do I know about poor Mother? Precious little. I know that she was Melody Nash. A beautiful name, promising so much. I know that she was born in Dublin and that she lived on Bolton Street. She worked in Mitchell's rosary bead factory on Marlborough Street. They made the beads out of cows' horns. All day, six days a week, sweating, going blind for God and Mitchell. Putting the holes in the beads for Jesus. Hands bleeding, eyes itching. " ~ A Star Called Henry - Roddy Doyle
Besides the regular factory workers, the poor of Dublin could work stringing rosary beads at home, receiving supplies from Mitchells and being paid tuppence for each finished decade they delivered.
“…And every evening Molly’s table became a rosary-bead factory. Cut and hook. Cut and hook. Even with gloves, the pliers and wire cut into your fingers. Cuts and calluses. Everyone had a go at making the rosary beads.” 
The style of these rosaries changed a bit over the years, but I have yet to see a vintage Irish horn rosary with a makers mark from any company other than Mitchells.
Horn rosary beads are no longer made in Ireland. I’ve run into many fanciful guesses as to why this is so, but the most likely reason is the smell of the boiling vats. These were a constant source of complaint in the neighborhood and were eventually banned from urban areas, sometime in the early 1960s. The Mitchell family is still in the rosary business though, making their rosaries from Connemara marble, glass, and plastic. Oddly, the family name is still used but the spelling has changed. It doesn't seem to be a typo as this spelling is used on every current Mitchel's tag. The logo and other information is the same, and I'm confident it's the same company. This rosary and box were made in the first decade of the 21st century:
1) Handbook to the city of Dublin and the surrounding district - British Association for the Advancement of Science; 1908 E. A. ilONTMOKENCY MoilRIS, M.A.
2) It’s a Long Way from Penny Apples’, Bill Cullen
Old Irish Rosaries ~ Edward A. McGuire
Published by: The Furrow, Vol. 5, No. 2 (Feb., 1954), pp. 97-104, 1-4, 105 (article consists of 13 pages)
The Goldenbridge Secret Rosary Bead Factory ~ Marie-Therese O'Loughlin Dec 27th, 2006http://www.butterfliesandwheels.org/2006/the-goldenbridge-secret-rosary-bead-factory/
Haunted by Irish rosary beads ~ James Carroll