Only the 40 Unionists took their seats in the 1921 House of Commons. Northern Ireland's Prime Minister proposed the measure ("that the powers of the Parliament and Government of the Irish Free State shall no longer extend to Northern Ireland"). There was no debate, with only one person speaking after and that only on a point of order. It was passed with no division, no vote count, just "Question put, and agreed to". (I wonder what the UK parliamentary equivalent is to "without objection, it is so ordered"?) The parliamentary record, et seq.

(Edited from a previous reply for rules purposes.)

In fact, most of what I can tell is that most of Northern Ireland had voted in 1918 for the Unionist party.

Northern Ireland Parliamentary Election Results lists the results of the 1921 election as

Party	Seats
Unionist	40
Nationalist	6
Sinn Féin	6

Early N.I. House of Commons records have the organization of Northern Ireland's House of Commons. 40 names taking the oath. 40 Unionists above. I didn't research the names, but back on p. 7 of the minutes: "I would even go further, and say that if the Nationalist Members of this House had felt it convenient to attend here to-day ..."

Sinn Féin refused to take seats in the [Southern] Irish Dáil for decades, much less any other parliament. (de Valera formed Fianna Fáil in 1926, with one goal to allow TDs to finally enter the Dáil.)

As for the Nationalists, R. F. Foster, Modern Ireland: 1600-1972, p. 529, says over the longer term "Catholic alienation became institutionalized. Their permanent minority status was reflected in the hopelessly small number of nationalist MPs elected, usually abstaining from taking their seats." I saw a reference elsewhere that the Nationalists boycotted the 1921 parliament, which fits the numbers above.