Patrick Collins - whose sister Sandra went missing in 2000 - says he 'doesn't really know how to tell people how it feels'
Today marks Ireland's fourth annual Missing Persons Day.
Every year gardaí receive around 9,000 reports of missing people across the country - with the majority of those cases solved.
However, other cases remain unsolved, with families left unaware of what happened to their loved ones.
An event taking place at Farmleigh House in Dublin's Phoenix Park will commemorate those who have gone missing and the trauma experienced by their family and friends.
The ceremony will include musical and personal tributes to missing people, and a tree will be planted in the Farmleigh grounds.
29-year-old Mayo woman Sandra Collins disappeared in December 2000, hours after she found out she was pregnant.
Her brother Patrick is convinced she died that night.
Speaking to Jonathan Healy on today's Pat Kenny Show, Patrick explained: "If I was to be brutally honest, I firmly believe that Sandra met with someone that night and that she was murdered simply because she was pregnant. And then they buried her secretly.
"I don't really know how to tell people how it feels - unless you're in the position that you're in, it's difficult to describe. You know they say time is a great healer and you move on - but I'm afraid with this time is no healer, and in fact the longer it goes the harder it gets, the more physically and emotionally draining it is.
"There's always the missing person there, and [we're always] wondering when we're going to find her," he added.