Bill Borland wrote:

No Sinn Fein are allowed. They refuse to do so because they believe taking
their seats would be recognising the legitimacy of the UK to govern
Northern
Ireland. (They used to boycott both the Dail and Stormont as well.) If
Sinn
Fein wanted to take their seats nothing could stop them.

Isn't it something about refusing to take an oath of allegiance?

They focus on that but that's only because it's easier to get public
sympathy for attacking Parliamentary protocol (and there is a long history
of opposition to particular oathes in Ireland) than pretending that by not
taking their seats they are undermining the basis of UK rule.