Yo, Cassie! Chels here, and I have the Irish Lord, Rowan Dempsey. He married Treasa, so he and Nico would know each other at least relatively well through the Byrne sisters. Rowan and Siobhan don't get along super well because lbr Rowan is purposefully not a great husband to Treasa, but he makes a point to be at least amicable to both Siobhan and Nicolas whenever he interacts with them, to kind of counterbalance the (generally very true) things Treasa shares with Siobhan.
And Karoline sounds lovely~ I suspect Rowan would like her when secret rebel-royals interact more with the rebels directly.