I learned today that the banjo was developed by Africans in the new world, but for me it's always been a traditionally Irish instrument. How did this happen?
The banjo (and indeed the guitar) have only really become accepted part of Irish trad music in relatively recent times.
The banjo as we know it today seems to have been created in the Caribbean and Southern US slave plantations based on earlier stringed lute like instruments brought over from Africa. The first known appearance of the banjo in Ireland was in the 1880s where it was played by Joe Sweeney of Virginia in a "minstrel" show (i.e. a "blackface" performance).
It seems to have started appearing as an instrument played by local musicians in the early 20th century, presumably imported to Ireland from by returning emigrants. It was seen as a fringe instrument, usually played in local dance halls to accompany an accordion during dances. It would not have been an instrument favoured by "serious" musicians who tended to focus on traditional instruments like pipes, harps etc.
When Comhaltas Ceoltoiri Eireann was formed in the 50s to promote and preserve traditional Irish music the banjo, specifically the 4 stringed tenor banjo, had grown in popularity as an instrument used in dances to accompany jigs and reels, Comhaltas were not particularly pleased by this development but couldn't do much about it.
The real boom in popularity came in the 1960s, during the folk revival the likes of the Clancey Brothers and Tommy Mackem were immensely popular in the US and they used the banjo to accompany their ballads, but it was really Barney McKenna of the Dubliners who led the growth of the instrument in the folk/ballad/trad scene that was building in Ireland.
McKenna was a skilled 4 string tenor player and his banjo playing was one of the keys to the Dubliners success, from the 1960s through to the 1990s the Dubliners were a huge draw in Ireland and around the world and became synonymous with "traditional" Irish music for many, so the style of the Dubliners was an influence on other bands and musicians who came up at the same time and after them and as a result the tenor banjo became a required instrument for any trad/ballad band.