"When I was starting out there was no course in Ireland so I had to study in Hereford in the UK for four years, but Ruairí was able to train in Kildare," Sam said.
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.
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.
"During the year farriers don't get to meet up and this is a great social event, as well as giving qualified farriers and apprentices an opportunity to compete against each other in a bid to raise standards," Alwyn said.
"Last year we had a mostly-male entry but we did have several women from the UK and one lady from Canada. It would be lovely to see more women involved here too, but it's slow to catch on."
In recent weeks, Alwyn McKeown was one two guest speakers at a seminar in Kildare organised by the European and Mediterranean Horseracing Federation (EMHF).
The EMHF represents 27 racing authorities and the main objective is to develop relations among European and Mediterranean counterparts, and to coordinate the promotion of horse racing in Europe and in Mediterranean countries.
A group of 22 delegates from Czech Republic, Germany, Sweden, Turkey, Slovakia, Denmark and Ireland were in attendance and among the topics discussed were shoeing techniques, ligament injuries and regulations and rules for the shoeing of thoroughbreds.
"Even though I was a guest speaker I actually learned a lot myself and it was good to chat to farriers from other countries to learn about new products and techniques."
Alwyn McKeown also served his apprenticeship in the UK but now runs a busy forge outside Lisburn.
"When I came home the standard wasn't great but I have since tried to build up a good service. I like to book everything into a diary, and keep note so that each client will be due a re-visit in six weeks' time.
"Not every farrier likes to do that but it's so important to become reliable."
Like so many professions, sadly there are a number of unqualified farriers operating under the radar in Ireland. It is up to each horse owner to ensure that their farrier has a recognised qualification.
"It would be good to see a legislation introduced to clamp down on those unregistered," Mr McKeown commented.
"I believe some insurance companies are requesting information on farriers in the case of certain claims by horse owners, so it is not before time," he concluded.