The original writeup in this node was by dunne, who is no longer with us (don't forget to write!), but I believe my writeup still makes sense. To contextualise a little, it is in response to the claim that the term "Irish Republican" refers only to those of the "physical force" strand of Irish nationalism.
Republicanism has been a political ideal in Ireland long before the IRA existed, and will be around after they are gone. That someone supports the ideal of an Irish Republic (normally on a 32 County basis) does not presuppose that that person also supports violence as a means of achieving this end. I would have to say that in general terms I support the ideal of a Republic (especially in the sense that it is the opposite of a Monarchy): this doesn't mean that I don't abhor the IRA and its ilk. I'm not a supporter of Fianna Fáil either, but you can't challenge their right to call themselves a Republican Party simply on the basis that they abandoned "physical force" a long time ago. They represent a far greater number of republicans than do Sinn Féin.
However, as well as Sinn Féin and the IRA attempting to set themselves up as the only true Republicans (and drag the rest of us down to their level), there is also the fact that in discussing Northern Ireland politics, "Republican" has become a convenient label for those on the Nationalist side who do use physical force. This takes in groups like the Provisional IRA, INLA, Continuity IRA etc. A similar "naming convention" exists on the Unionist side: those within unionism who use violence, such as the UVF and UDA, are termed Loyalists.
These labels don't really mean much, and can be confusing for the outside observer, but they make the lives of those in the media easier. I have found that foreign correspondents tend to simplify things even further, by referring to Protestant and Catholic groups. All in all, these terms are not to be taken too literally, they are simply convenient labels employed by commentators in a perhaps foolhardy attempt to make sense of the whole mess. I have to admit that I use them myself quite regularly in my writeups.