Should elected Members of Parliament in Northern Ireland be able to sit and participate in the Dáil?

Yes but not vote.

I don’t know enough about this

No but members of both parliaments should meet regularly to discuss and co-operate on island wide affairs

In aid of unity yes. In the present circumstances of Northern Ireland and the republic being separate, no

Without the context of how it would impact dail operations or constituents either side of the border, this is a difficult question to answer. If it's with a view to a united Ireland, then absolutely yes.