That’s… Interesting. Here in the UK, Sinn Fein does the same, with their elected MPs refusing the oath and abstaining completely from sitting in the House of Commons. Different outcome, however, as Northern Ireland only has 19 out the of 649 seats in the HoC.
(The other nationalist party in NI, the SDLP, reluctantly take the oath. Their MPs currently sit on the government benches with Labour, as is traditional.)
Wondered how the Northern Ireland assembly works, as Sinn Fein participate in government there, and apparently it doesn’t require electees to take an oath to the monarch to take their seats or hold ministerial seats. I assume, without doing more investigating that the Good Friday agreement had clauses to allow that.