Background: Family members, or others, often assume the role of informal (unpaid) carers of people with chronic illnesses. Care-giving, however, can impact profoundly on the quality of life of carers and can cause carer worry, stress and guilt. Implementing interventions that positively affect the lives of carers is important; however, carers as a group are often difficult to reach. We embedded a study within a pilot-feasibility trial of a mindfulness based intervention to determine and prioritise the key motivators and challenges influencing informal carers’ decisions for participating in a trial. Methods: We used a multi-method approach involving interviews with participants from a ‘host trial’ and data from systematic reviews to develop a survey that was distributed to informal carers in Ireland. The survey consisted of 28 motivator and 17 challenge statements. Participants rated how important they thought each statement was when deciding to take part in a trial on a 5-point Likert Scale. Mean scores and standard deviations were calculated for each statement and arranged in descending order to provide the priority lists. Results: Thirty-six carers responded to the survey. Helping to create awareness about carers was the top ranked motivator, followed by four study design statements related to the time at which the study occurs, the study location, format of delivery and venue. The least important motivator related to how carers were invited to take part in a study. Difficulties in planning due to the caring role emerged as the most important challenge, followed by being unable to leave the care recipient on his/her own. Conclusions: Insight into decision-making for research participation will assist trial developers tailor trial processes for informal carer populations. We recommend that trialists should consider these motivators and challenges when designing future trials involving informal carers so as to enhance trial feasibility and success.