It refers to members of the former Parliament of the Kingdom of Ireland becoming members of the Parliament of the United Kingdom when the UK was created in 1801.

It refers to members of the former Parliament of the Kingdom of Ireland becoming members of the Parliament of the United Kingdom when the UK was created in 1801. Got it. Co-opt as in absorb, take-over, appropriate. Were all Irish Parliament constituencies co-opted or only selected?

Looks like it depended on the constituency: "All British MPs and those Irish members representing constituencies which retained two members after the Union (the 32 Irish counties and the cities of Dublin and Cork) automatically retained their seats when the Union took effect on 1 January 1801. "Those members of the Irish House of Commons to sit at Westminster, who represented constituencies still enfranchised after the Union but reduced from two members to one (the remaining 31 most considerable Irish boroughs and Dublin University), were selected by drawing lots. If one of the seats in the Irish Parliament was vacant, then the remaining member for the constituency was automatically chosen for the Westminster Parliament. If both seats were vacant a by-election was held." https://en.wikipedia.org/wiki/First_Parliament_of_the_United_Kingdom#Dates_of_co-option