Sayer's last cap came before he joined the Albion. Peter O'Sullivan played 3 times for Wales; he did play for them against Scotland in the Home International on May 6 1976, but 8 days later he wasn't in the side that faced Northern Ireland (with Albion's Sammy Morgan) at the Vetch Field.
Depends whether you consider the Home International Championships a "competitive" fixture, I guess. They did indeed start against each other at Wembley on February 23 1982.
Not sure about a competitive fixture, but Gordon Greer and Aaron Hughes played against each other for Scotland and Northern Ireland respectively in a friendly at Hampden Park on March 25 2015.