There is a historical context to this. Irish MPs have either withdrawn Westminster, abstained in votes, or acted to obstruct the processes of Parliament since 1845.
Prior to the formation of the Irish Republic
The Irish Republican Brotherhood refused to engage with the UK parliament by having imprisoned felons elected. Others, in the Home Rule League proposed that Irish MPs should meet in an alternative parliament.
After 1916 the Irish Parliamentary Party did take their seats, while Sinn Fein did not. The question of taking seats in the Parliament of the Irish Free State. Some did, and some did not (as they did not recognise the legitimacy of the Irish Free State)
Sinn Fein has been through various splits. The offical party evolved to a workers party (far left wing) and has largely disappeared. While a provisional party, associated with the Provisional IRA, contested elections while refusing to take their seats. The Modern Sinn Fein party evolved after the electoral success of Bobby Sands, who won his election while in prison and on hunger strike. Party conferences in the early 1980s re-affirmed the principle of refusing to take seats in Westminster.
The Parliament in Stormont doesn't require members to swear allegiance to the Queen, to allow Sinn Fein to join. Instead there is a pledge of office (promising to act in good faith, and renouncing violence)