Dan O'Brien biography Dan O’Brien is a professional musical theatre actor based in the UK. He is currently working as an ensemble in the latest production of Mamma Mia! Training: He went to the London School of Musical Theatre. Theatre: Among his credits in theatre is The Best Little Whorehouse in Texas at the Union Theatre; On A Clear Day You Can See Forever at the Union Theatre; Dreamboats And Petticoats in the Number 1 UK Tour and Jersey Boys at the Piccadilly Theatre. TV: Dan appeared in Let It Shine, a Jersey Boys performance. Workshops: He also attended the workshops for Vagabond Skies at the Leicester Square Theatre and Being Ivor at the Lost Theatre.