Translation:Your father used to steal but now he is in prison.
Do we really need the article with "prison" in the English? Unless referring to a specific instance of a prison, I think "in prison" is more frequent. Pota Focal translates an example of "sa phríosún", from beo.ie, as "in prison" and teanglann,ie has " Cuireadh chun an phríosúin é" translated as "he was sent to prison".
That differentiation doesn't really exist in Irish (nor in English in Ireland). The NEID and EID entries for "gaol" and "jail" both give príosiún.
The main prison in Dublin is officially "Mountjoy Prison", but is commonly referred to as "Mountjoy Jail", and historically, "Kilmainham Gaol" was used to house people who had been convicted of serious crimes, and Oscar Wildes "The Ballad of Reading Gaol" was based on his sentence of 2 years hard labour.
No, they built a prison in a place that was already called Mountjoy - Viscount Mountjoy (apparently named for a Mountjoy in Co Tyrone) was the MP for Dublin in the late 18th century, and he developed Mountjoy Square, on the north of the city. The prison was built nearby 30 or 40 years later.
The prison is colloquially referred as "the 'Joy". And the Dóchas Center is on the same campus.
Weird that (again - happened on another exercise) I wrote the exact sentence they give, yet they give the same sentence as "another translation."
I wonder if it's because the "correct" answer changed from "the prison" to just "prison" and this is somehow a bug... Doesn't really make sense but it's all I can come up with.