The 18-year-old Guiu took over as the competition's leading scorer with six goals, two of which came in the win away to Astana last week.

The young Spaniard headed Chelsea in front after a defensive mix-up but a deflected effort from Markus Poom -- the son of former Derby, Sunderland and Arsenal goalkeeper Mart -- brought the Irish visitors level.

Guiu took advantage of another mistake at the back to restore Chelsea's lead at Stamford Bridge, with Kiernan Dewsbury-Hall adding another before Guiu completed his treble by heading home in first-half stoppage time.

Marc Cucurella fired in a fifth for the Blues around the hour as Chelsea finished the league phase with 26 goals scored -- eight more than any other club -- and again underlined their status as the title favourites.

"A lot of people will say we are big favourites and honestly, we probably are, but in football it is not easy to keep winning," Dewsbury-Hall told TNT Sports.

"Six wins out of six is pretty good.

"This tournament is relieving a lot of pressure for some of the people who play in the Premier league and it's a chance for young academy lads to make their debuts.

"It is a good vibe at the moment."

Chelsea coach Enzo Maresca admitted Guiu was frustrated by his limited opportunities, having not played a single minute in the Premier League since a substitute appearance in the opening game of the season.

"Marc is a bit unhappy as Nicolas (Jackson) and Christopher (Nkunku) are doing well," said Maresca.

"When you're a nine and another nine is doing well it is a matter of being questioned. It is important that they work hard day-by-day and then they get a chance."

Shamrock, who began the night on course for the top 16 as one of only three remaining unbeaten teams, will instead head into the draw for Friday's knockout phase play-offs among the seeded sides.

Joining Chelsea in qualifying directly for the last 16 were Vitoria Guimaraes and Fiorentina after a 1-1 draw between the sides in Portugal. Vitoria were the only other team not to lose in the league phase.

Rapid Vienna, Djurgarden, Lugano, Legia Warsaw and Cercle Brugge will also go through to the last 16.

Hearts missed out on a play-off spot after a 2-2 draw at home to Romania's Petrocub, who came dead last among the 36 clubs in the competition.

Northern Ireland's Larne had already been eliminated but picked up their first points with a 1-0 win over Gent, while Welsh outfit The New Saints lost 3-2 away to Slovenia's Celje as their European run also ended.