I don’t think they ever said. Since she’s a devout Catholic, I assume she wasn’t divorced and that he must have died.
.
I assumed she was a widow. Of course it is possible that she seperated from her husband without divorcing him (was divorce even legal in Ireland when that show started? 
). That’s perfectly acceptable to the Church as long as both neither spouse has sex with anyone else.
I always assumed he drowned.
In tea.
I like Alan Smithee’s answer.
Ah, go on go on go on go on go on go on…
I have also heard another possible explanation. Apparently (I have no cite) it is/was not uncommon in rural Ireland for an unmarried woman past a certain age to be given “Mrs” as a courtesy title.
Compare and contrast the common practice (certainly in Australia) for classical ballet teachers to be called “Miss” and their first name. In other words Jane Smith would be known as “Miss Jane” regardless of her marital status.
We should think of Mrs Doyle as a serial killer with a very inefficient MO.
I’m remembering also UPSTAIRS, DOWNSTAIRS where the cook was clearly single but was called “Mrs Bridges” as a courtesy title. It’s one of those cute British things, just as the butler is called by last name (“Hudson”.) We always just assume that a similar phenomenon happens in [del]Ireland[/del] Craggy Island?