Due to the introduction of animal welfare bye-laws by Dublin City Council, Smithfield Fair is also among those now set for extinction.
Once held monthly, now only two fairs – in March and September – remain.
Sadly, despite ongoing campaigns of support from local owners, it appears their days too are numbered.
And although Athenry Fair has already been lost, the popular Castleblakeney Fair is among six or seven now which remain in Galway, along with Maam Cross, Aughrim and Kilconnell.
However, like his fellow horse traders, Killeen is no longer living in an era when transport was an issue and now he will travel the length and breadth of Ireland in a bid to make a sale, or find that special bargain.
"I will travel all over the country once I have transport," he says, adding that Tallow Fair in Co Waterford is also among his favourite haunts.
As with Ballinasloe, Tallow Fair, which takes place on September 3 each year, attracts some of the biggest horse dealers in Ireland, including Jim Derwin and Miley Cash.
It is no secret that both men are capable of buying anything between 30 and 40 horses and ponies at any one fair, particularly from Tallow, Spancil Hill (Co Clare) and Cahirmee (Co Cork). Stock at their yards in Athlone and Monasterevin, respectively, can reach into the hundreds at any time.
Jim Derwin, along with his brother Francis, has been buying and selling horses for well over 20 years and during that time has also seen the horse trade undergo radical change, particularly in the last few years.
"Owners are under a lot of pressure at the moment and are mad to sell stock to get some money to support themselves," he says.
"Many of them cannot afford to keep their animals anymore and are selling at prices considerably lower than they would have five or six years ago.
"Business for us has been good of late , and we have been lucky to find quality animals at the right price.
"However, horses are still being offered without passports and this continues to be an issue," Jim adds.
"We mostly sell to the UK, but also mainland Europe. Last year we saw a drop in British buyers for hunters but that was due to the bad weather which resulted in the loss of a lot of hunting days.
"Now the demand for good Irish hunters is coming back and I was delighted to find a super one at Cahirmee in July."
Derwin agrees that finding a gem can often be down to pure luck on the day, but it is not often a dealer can boast at having sourced two Olympic event horses in Mr Medicott and Electric Cruise in recent years. It is an eye for a good horse that sets these men apart.
The same can be said for Miley Cash who can count Badminton winner Star Appeal and top show jumper Cullawn Diamond among his former residents.
Aside from these special purchases, Cash rarely misses a horse fair and from Galway to Cork, and Clare to Kerry, he has attended practically every fair in the country during his 60 years in the business.
"Things are a lot different this year though," Cash maintains, having seen a dramatic drop in horse trade at the Puck Fair in Co Kerry last month.
"This year at Puck there were no buyers down from the North or over from Britain and trade was very poor, but yet Spancil Hill in June was up on previous years and I bought 53 that day," he recalls.
"Owners are getting fed up travelling around to fairs and not getting buyers for their animals so that is why some fairs are seeing such a drop in numbers," he says.
With Ballinasloe Fair the highlight of the year for Cash and many others, it is hoped that business will be brisk, yet Cash remains cautious.
"I worry that this fair is too late and people will not be buying as many horses as usual as we come into what will be another lean winter."
For the sake of those intending to sell at Europe's oldest fair, it is hoped that, on this occasion, Cash will be proven wrong.