Been watching Downton Abbey and someone is always getting asked about (or accused of) being a republican.
It sounds like it's another word for 'radical or 'anarchist', but I feel there's an actual political aspect I'm missing. (You think I'd know this, as my grandfather was in the IRA, but I find myself pretty ignorant here.)
Oh, also:
Irish republicanism (the sort advocated by the IRA) is a totally different radical political position: the position that Ireland should split from Britain and become a republic.
Since most of Ireland did that already, the 70s-era IRA instead held the position that the parts of Ireland-the-island that are currently part of the UK should instead become part of Ireland-the-country. At the time of Downton Abbey, though, Ireland was still entirely part of the UK.
In the context of British politics, a republican is someone who wants to abolish the monarchy and make the UK into a republic.
It's a fairly radical political position, at least in Britain (it's much more popular in some other ex-British territories, like Australia). Accusing someone of being a republican in Britain is very roughly equivalent to accusing someone of being a socialist in America.