Good piece if trivia. Decided to do the research and unsurprisingly the answer is yes, he is the only one.
Full list if ROI player involvement in European finals here (started the match(es) unless otherwise stated):
1968 Shay Brennan, Tony Dunne (Man Utd EC winners)
1971 John Dempsey, Paddy Mulligan *came on as sub in first game, unused sub in replay (Chelsea ECWC winners)
1972 Joe Kinnear (Tottenham UEFA Cup winners)
1973 Steve Heighway (Liverpool UEFA Cup winners)
1975 Johnny Giles (Leeds Utd EC runners-up)
1976 Steve Heighway (Liverpool UEFA Cup winners)
1977 Steve Heighway (Liverpool EC winners)
1978 Steve Heighway *came on as sub (Liverpool EC winners)
1980 David O'Leary, Liam Brady *missed in shoot-out, Frank Stapleton *scored in shoot-out (Arsenal ECWC runners-up)
1984 Mark Lawrenson, Ronnie Whelan, Michael Robinson *came on as sub (Liverpool EC winners)
1984 Chris Hughton, Tony Galvin (Tottenham UEFA Cup winners)
1984 Mickey Walsh (FC Porto ECWC runners-up)
1985 Jim Beglin, Mark Lawrenson, Ronnie Whelan (Liverpool EC runners-up)
1985 Kevin Sheedy (Everton ECWC winners)
1991 Denis Irwin (Man Utd ECWC winners)
1994 Eddie McGoldrick *came on as sub (Arsenal ECWC winners)
1995 Eddie McGoldrick *unused sub (Arsenal ECWC runners-up)
1999 Denis Irwin (Man Utd CL winners)
2005 Steve Finnan (Liverpool CL winners)
2007 Steve Finnan (Liverpool CL runners-up)
2008 John O'Shea *unused sub (Man Utd CL winners)
2009 John O'Shea (Man Utd CL runners-up)
2010 Damien Duff (Fulham EL runners-up)
2019 Caoimhín Kelleher *unused sub (Liverpool CL winners)
Useless info:
We've never had players on opposing sides in a European final
1984 was our bumper year, with six player involvements across the three competitions)