Both sides have produced some memorable performances so far in the latest global gathering, with Wales running South Africa close and hammering Fiji while Ireland stunned Australia and made light work of Italy.
Quinnell, who was a Lion in 1997 and 2001 and featured at the 1999 World Cup, has been impressed with what he's seen from his countrymen in Pool D but he admits that the nature of Ireland's recent performances has him a little worried.
"When I saw the Wales game against Fiji I thought 'Yes, that's outstanding', and then I sat down to watch the Ireland game against Italy and thought, 'Ooh, they're looking good', with the likes of (Sean) O'Brien, (Jamie) Heaslip and (Stephen) Ferris there," Quinnell told Talk Sport.
"Ronan (O'Gara) came in and did a wonderful job and (Brian) O'Driscoll came in and scored that all important try and you just think that they really are peaking at the right time.
"In August you felt as though a couple of their guys were a little bit over the hill but they have trained through that period and really have hit their straps at the right time, coming into the quarter-finals of the World Cup."
While it is the experience of Ireland's veteran Lions tourists and seasoned internationals that have largely led the Irish challenge in New Zealand, the youngsters have taken on that role with the Welsh.
Warren Gatland's decision to place his faith in youth appears to be paying dividends and Quinnell believes that the Wales coach now has plenty of options at his disposal, especially with James Hook, Shane Williams and Dan Lydiate back to full fitness.
"A lot of young boys are playing so well, the likes of George North and Scott Williams who came into the team last week and played like he'd been there all his life," added Quinnell.
"Jamie Roberts is coming back into his Lions form now and Rhys Priestland looks like he's taken over from Stephen Jones as he's playing with so much confidence.
"There's Leigh Halfpenny on the wing, Shane Williams to come back in, Lee Byrne did ok in the last two games, is James Hook going to get back in? It's a wonderful problem for Warren Gatland to have."