Drawing a blank here. I know how to say that one person is the oldest and another the youngest in a family, but how would you say that someone is the brother or sister in the middle?

Redwolf

On Foclóir.ie they have - the middle son - an mac sa lár, an mac i lár báire, or - an mac i lár baill.

GRMA

There's this as well - his second youngest son -an dara mac is óige aige.