The 23-year-old brings with him a pedigree built at some of the country's better-regarded academies. He came through Manchester City's youth ranks, where Pep Guardiola named him among the substitutes on five separate occasions, including for a Champions League night against RB Leipzig, before a loan at Rochdale under former Barnsley defender Jimmy McNulty gave him his first taste of senior football.
Slicker moved to Ipswich in July 2023 in search of regular first-team football, and spent last season on loan at Barnet, keeping 15 clean sheets across the campaign. He now becomes the latest addition to Daniel Stendel's squad ahead of the new season.
"After last year, the next step was key for me, and I think this is the perfect step," Slicker said. "It's a massive club with a rich history and that's what I want. I want to play in front of great fans in a stadium like this."
Stendel said the club were pleased to have brought in a goalkeeper of Slicker's profile.
"We're very happy to bring Cieran to Barnsley," the head coach said. "He is a goalkeeper with excellent quality, good experience for his age and a strong mentality. He has worked in top environments and shown his ability during his loan spell last season.
"Standing at 6ft 3in, he is a commanding presence in the penalty area and a modern goalkeeper who is very comfortable with the ball at his feet. We believe he will fit our style of play very well, be an important player for us, and we are excited to work with him."
Born in Oldham but qualified for Scotland through his Glaswegian father, Slicker has represented his adopted country at every youth level before earning a senior call-up in November 2024.
He made his full international debut against Iceland last June, coming on in the seventh minute to replace the injured Angus Gunn. He remains under contract at Ipswich until 2028, meaning the move to Oakwell is a loan rather than a permanent departure from Portman Road.