The siblings are based in Askeaton and cover Limerick, but also parts of Clare, Kerry and Tipperary. Meanwhile, their cousins John, Padraig and Liam Brennan run an equally thriving business in nearby Adare.
"With the recession eight years ago the first luxury to go was the horse, and our business suffered as a result, but things are good again and we are kept busy year-round," Sam added.
In addition to some sport horses, they mostly specialise in the racing industry, between racehorses and preparing young horses for the sales.
Trade
While a handful of women in the UK are now learning the trade, Sarah Callow is believed to be the only female farrier currently in the business in Ireland. She studied at Oatridge College in the UK and served her apprenticeship with Irishman John Dooley, whom she met as a teenager in her native Isle of Man.
Now based outside Dunlavin, Co. Wicklow, she covers a wide area between West Wicklow and Kildare, with some other long-term clients further afield.
Without doubt life as a farrier is a tough and it's often a thankless job, but it is thoroughly satisfactory when things go right. "Some young men think that it's a great job, and so it is, but it is also very tough physically".
"It's very hard on your back especially," said Alwyn McKeown, who next Saturday hosts the ninth annual Crossan Forge Farrier Competition in Antrim.
It is the only such competition held north of the border and each year attracts entries from across Ireland, the UK and beyond. Sam Brennan is among those who have picked up numerous wins in recent years.
Sam Brennan runs a farrier business with his brother Ruairí in Askeaton, Co Limerick