I'm going to have to disagree with you here. I would never, ever call my Mum "Mam" it just sounds wrong. A lot of people do use that but it is, overall, a small percentage of the population. I only know about two people who call their Mums that. Seamus is very Irish but also very stereotypical character. He represents Northside Dubliners and to some people can be seen as quite annoying and unrealistic (not to me though, I love Seamus!)
Generally, they call their mother's "Mam" and sometimes repeat themseleves a bit. If you look at Sheamus in the books, there is no dialect apart from reffering to his Mum as Mam, and his is pretty obviously Irish.