Ir·ish jack·ham·mer ahy-rish jak-ham-er (Verb)
The act of a man with a small penis pounding away mercilessly at a woman's vagina in attempt to get deeper.
(Noun) Small penis of any nationality.
I was having sex with this big girl. I had to give her the Irish jackhammer for her to even feel it!
When a man (or woman) inserts their fist into a woman's vagina or anus (also could be a male's anus) and proceeds to violently shake them, while still inside, potentially causing extreme vaginal or anal trauma and/or Downs' Syndrome.
Samantha: Did you hear about Jeni? I heard Alex gave her a gnarly Irish Jackhammer!
Ryan: No shit! What a fucking whore!