I had 4 interviews with Mercer with 6 employees (4 in Dublin, and 2 in the US). The whole interview process was quite long, it took 6 weeks from the first interview to the offer. The last two interviews were more on chatting instead of proper interviews. The overall experience was quite good.

Thorough and lots of situational questions. Free flowing conversation between the interviewers and me. Asked a the normal questions - why are you looking to leave, what are your goals in your next role etc.