So this new instrument, the Violin, arrived in Ulster around the 17th century & the great movement of people from Scotland to Ulster at that time probably speeded up the Violins spread, especially in the country areas where the planters settled.
Alan Jabbour in his 'Fiddle Tunes of the Old Frontier' lecture says: 
"The favoured instrument for this instrumental music revolution was the modern Italian-style violin, which had spread northward through Europe in the 17th century. In the 18th century a number of manufactories for making violins sprang up in Germany, France, and England, and they had the effect of democratizing the violin, making it available not just to professional musicians but to people from all walks of life. So I am inclined to attribute the revolution in instrumental music to the advent of the modern violin, as a widely diffused new instrument, combined with the general revolutionary spirit of the later 18th century in the British Isles and colonies."
James Christie mentions the Sligo Fiddling Style & points out that it is very different to the Co. Antrim style, with all it’s Scottish influences & the paper debates the influence of the Scottish style of Fiddling on the Co Antrim style, & it is clear to see how the Fiddlers in that county would have been heavily influenced by Scottish Fiddling, but I would suggest that, as each of the Irish border counties has it's own individual style, especially Donegal, so each of Ulster's border counties fiddlers would have been influenced by their neighbours style across the border.
A fellow debater on a discussion board I visit, who goes by the handle of 'Harry', & whose opinions I respect, interestingly maintained that:
"Sligo was a busy port for Scots traffic in the 19 th century and it is thought that quite a lot of music came in that way. This may account for some or much of the Scots material that plays a big role in Sligo repertory... and it may not. There was a lot of Scots music about on paper besides, and huge amounts of seasonal and other migration between Donegal (just up the coast) and Scotland. It is just as concievable that Scots material came to central and eastern Ulster from the west in cases. This is certainly the case of western ifluence on Leitrim and Fermanagh where ideas of political borders did not seem to effect the distribution of music."
So it clearly isn't all as simple as tunes wafting across the Irish Sea to East Antrim & slowly diffusing through the populace from there.
Local radio would play a part in more recent years, for I have no doubt they would promote their own areas musicians, so that local style would be the one musicians would hear, so an Ulster-Scots Fiddler in say Co Fermanagh would be more likely to have been influenced by Donegal, or Leitrim styles than Scottish styles, but Scottish styles would have been around for them to hear, too.
It should also be noted that, while large groups of Fiddlers are being taught to play by likes of the 'Fiddlesteeks' & 'Antrim Fiddlers' groups, it must be remembered that they are being taught by classical musicians who may, although enjoying the traditional tunes, have a fairly poor understanding of what actually makes a good 'country fiddler'.
However, it must be hoped that once many of these young fiddlers break away from their fiddle orchestra settings, they can blossom as traditional musicians, by mixing with local musicians & absorbing the tunes orally & developing a relaxed & natural country style of playing. So anyone teaching the fiddle, by whatever method, has to be applauded.