Her mother has since told her stories about her Dad in his better days, but at the time she was too young to grasp the gravity of the situation - and too young to remember her father before he became ill.
While Alzheimer's is associated with old age, the disease can also strike early, affecting people in their 30s, 40s and 50s.
What's more, cases of young and early-onset Alzheimer's are on the rise, with researchers from Bournemouth University describing it as a "silent epidemic".
The initial signs of early-onset Alzheimer's can be harder to detect in younger patients as the disease tends to manifest in more unusual ways.
For Niamh's mother, Catherine, who was 32 when her husband was diagnosed, the change in Liam's personality was the first indication that something was seriously wrong.
Little by little, the other signs became more apparent.
Liam, who was a school teacher at CBS Nenagh, started misplacing the car keys and forgetting to correct his pupils' homework. More distressing still was the day he collected Niamh and her older brother Colm from school and forgot his way home.
"Apparently, we had to show him the way back," says Niamh. "I thought he was joking, but he had absolutely no idea what was going on. I look back now and think that must have been so scary for him."
Liam's age made it difficult to get a diagnosis. While their local GP was hugely supportive, Catherine struggled to find a specialist who would take her concerns seriously.
After a long and tedious process, she went to Queen's University Hospital in London where a blood test was used to diagnose the disease. Liam was 42.
"People didn't believe her and doctors told her she was imagining it," explains Niamh, before adding they were lucky their mother persisted - otherwise they might not have received the help they desperately needed.
Liam retired from his job during the same period and, with a young family to look after, Catherine was forced to become the main breadwinner.
Meanwhile, Liam's mental faculties began to deteriorate rapidly and two years after his diagnosis, Catherine had to make the decision to move him into full-time residential care.
"He kind of just lost himself - that's the best way to describe it," says Niamh.
"With Alzheimer's, it's harder for the family. The person, in my experience anyway, doesn't even realise it's happening." While some Alzheimer's patients can become aggressive and agitated, Liam was calm and composed throughout his illness. It was a small mercy for which the family are still grateful.
"He was always a very gentle person - the only way my mother describes him is as a gentleman, which I love," says Niamh. "He was always very calm and Mam said he was never angry going through the mood changes as he got sick. Confused, but never angry."
During the earlier days of his illness, Niamh looked forward to visiting her father in the nursing home. However, as she reached her teenage years and his condition worsened, it "became too hard to watch".
At this point, Liam was bed-ridden and non-verbal and he had difficulty swallowing and digesting food. It was a distressing sight for his teenage children and by the time Niamh turned 13, it had become almost too much to bear.
"Everything avalanched," she says. "Before that, I understood that my Dad wasn't able to stay at home, that he wasn't well and he probably wasn't going to get better - but then I started to realise that it wasn't normal.
"I developed depression and it gradually got worse as I got older."
It was during this period that Niamh began to consider the possibility of developing the disease herself.
Early-onset dementia is more likely than late-onset dementia to be hereditary and, in the two years following her father's diagnosis, Niamh's aunt and uncle - Liam's older sister and younger brother - were also diagnosed with the disease.
"His father got senile very young," explains Niamh. "When I think back, I assume it was the same disease.
"For years, especially as a teenager, I was obsessed with the idea [of getting tested] and I really, really wanted to know, but it's funny, nobody knows what's happening to them and tests aren't always right. You might have a gene, but it might not be active.
"I think, for the time being, I'm very happy not knowing because I don't think it would change anything for me. If I want to go on to have a family, I will reconsider."
When Liam was diagnosed, Catherine brought her children to Rainbows - a support programme for young children who have been affected by loss due to bereavement, separation or divorce.
"My brother and I went into different groups because of our ages, but I remember I was the only person with that problem," she recalls. "Some people had divorced parents or different things going on at home, but nothing near what we were going through.
"The major complication as a teenager was that we were grieving for someone who was still there, so that conflict of it not seeming right or fair to that person was one of the major struggles," she adds.
Niamh says Rainbows gave her the coping skills to negotiate the grieving process and she urges anyone with young children who are affected by Alzheimer's to consider attending.
Nowadays, the Nenagh-born model uses talk therapy to help deal with her anxiety and depression.
"I've pretty much spent the last nine years going between different therapists," she says. "My Dad's illness added an extra complication to my teenage years and, as a family, we all had our own struggles. I think everything started from that - therapists have told me that going through a traumatic experience so early in life adds a lot of complications.
"Besides, it's hard to talk to one another and I know that we still struggle with it. For years, we couldn't talk about it at all, so having that outlet is important. Everyone needs someone to talk to."
Niamh thinks of depression as something she can manage, as opposed to something she can beat. Suffice to say, there are peaks and troughs.
A bad bout of depression in 2014 led to her dropping out of an Arts degree in UCC.
"It was around the time of my Dad's anniversary and I wasn't able to keep up with the course," she explains.
"I needed that time to figure out myself and go where I needed to go.
"Modelling work was something I was at for a few years and really passionate about. The work really started to pick up and then I stumbled upon the Miss Tipperary competition and everything fell into place."
After winning Miss Tipperary, Niamh was crowned as Miss Ireland 2016 last September. She dedicated the win to her late father.
"It's been an incredible experience," she smiles. "I still go through a lot even now, but I'm doing well."
* Niamh Kennedy is an ambassador for Alzheimer's Tea Day 2017, which is proudly supported by Medtronic.
Thursday May 4 is Alzheimer's Tea Day, the annual fundraising campaign for the The Alzheimer Society of Ireland. All funds raised locally stay locally.
For more information or to register for your fundraising pack, visit www.teaday.ie or call 1800 719 820