Slicker began his career at Manchester City, being named on the bench on numerous occasions under Pep Guardiola. 

In July 2022, Slicker signed for League Two side Rochdale on loan, making three appearances, two in the EFL Cup and one in the EFL Trophy, before being recalled in January 2023 by Manchester City. 

In July 2023, the goalkeeper signed for Ipswich Town for an undisclosed fee, signing a three-year contract. In October 2024, Slicker signed a new deal lasting until 2028. 

Slicker joined Barnet on a season-long loan on 7 August 2025. The 23-year-old made League Two appearances keeping 15 clean sheets.  

The goalkeeper has one senior Scotland appearance, and was capped at under-17, under-18 and under-21 levels. 

Slicker told the club website: “After last year, the next step was key for me, and I think this is the perfect step. 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.” 

Head coach Daniel Stendel added: "We're very happy to bring Cieran to Barnsley. 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 six foot three, 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."