David Cantrell wrote:

No, they're not allowed to. That's because to take your seat at
Westminster you need to swear an oath of allegiance to the queen. If
they were to do so they would obviously be lieing.

Wrong. Avowed republicans can and do take the oath - the late Tony Banks was
notorious for doing so with his fingers crossed and in the Scottish
Parliament the Scottish Socialists did so with protests as well. Mark
Durkan, the SDLP leader, is another avowed anti-monarchist and he takes the
oath. That none of them have ever been charged shows how it is not a
barrier.

Irish republicanism is not just about replacing the monarch with an elected
head of state but about creating a separate independent Irish state. Irish
independence originally began because the Sinn Fein MPs elected in 1918
boycotted Westminster and set up their own Dail. The modern day Provisional
Sinn Fein maintains this tradition.