Leinster needed two late tries to see off Bayonne in their final Investec Champions Cup pool game but Ireland players Tadhg Furlong, Robbie Henshaw and Tommy O'Brien all suffered injuries.
Sam Prendergast and Max Deegan both crossed after Leinster, now undefeated in 31 pool games, trailed with just six minutes to play in the Basque Country, against a side that hadn't managed a match point in three previous European games.
The four points means Leinster topped Pool 3 but Leo Cullen's side will have to wait to see how the rest of the weekend's games play out, with Glasgow against Saracens and Bristol v Bordeaux of most interest, to see how high up the seedings they finish.
Both games take place tomorrow.
With the Six Nations just over two weeks away, Ireland head coach Andy Farrell, who is already low on front row options, will be hoping Furlong's calf injury is not serious.
With Leinster having done most of the heavy lifting in earlier wins over Harlequins, Leicester and La Rochelle, this was a game devoid of much jeopardy and that was noticeable in the early exchanges in which the home side looked more lively.
Despite having nothing to play for in Europe, Grégory Patat's side have a proud home record and are unbeaten in the Top 14 here since April 2024.
The visitors' discipline was once again an issue, conceding nine penalties in total, and Bayonne opened the scoring off the back of Tommy O'Brien's illegal aerial challenge on full-back Cheikh Tiberghien.
Quick ball off the subsequent lineout saw Yohan Orabe come off his wing and combine with centre Sireli Maqala, who raced under the posts in a worryingly easy concession.
Before Harry Byrne was able to restart, Furlong was replaced by Thomas Clarkson with reports coming through soon after that the prop had a tight calf.
Joshua Kenny and Jimmy O'Brien were counter-attacking to good effect and after Kenny was denied in the corner, Byrne tapped over a penalty to get Leinster up and running.
However, that was cancelled out moments later when out-half Joris Segonds struck a long-range penalty after a lineout infringement, while Henshaw departed with a knee injury six minutes before the break.
After Clarkson was denied a score due to a double movement, Bayonne had two chances close to the end of the half but Deegan came up with a big tackle to deny Herschel Jantjies and the hosts went in 10-3 ahead.
Tommy O'Brien was next to pull up, the Ireland winger appearing to damage a calf injury at the start of the second half.
The game was scrappy and Jantjies was in the bin for a deliberate knock on when replacement Dan Sheehan (below) ran onto Jimmy O'Brien's offload to score with Byrne's conversion squaring up the tie.
But Segonds needed no second invitation to restore the lead, striking a penalty from his own half after another breakdown infringement.
Leinster looked to have taken the lead when Jimmy O'Brien collected Sam Prendergast's clever dink into space but, after the conversion had been taken, the TMO alerted referee Luke Pearce to a fumble before the touchdown.
The Bayonne defence was strained but did enough to hold on and Segonds was off target with another penalty from distance.
The four-time champions attacked on the 22 but appeared to be going nowhere before Byrne's perfect chip bounced up for Prendergast who scored under the posts.
Replacements Joe McCarthy and Scott Penny helped win a midfield penalty that paved the way for the try to seal the win.
Sheehan took advantage of an overthrow and Deegan dived over in the corner when the ball was recycled.
LIVE SCORING, FIXTURES, RESULTS AND STANDINGS
SCORERS:
Bayonne - Tries: Sireli Maqala
Con: Joris Segonds
Pens: Joris Segonds (2)
Leinster - Tries: Dan Sheehan, Sam Prendergast, Max Deegan
Cons: Harry Byrne (2)
Pen: Harry Byrne
Bayonne: Cheikh Tiberghien; Tom Spring, Sireli Maqala, Manu Tuilagi, Yohan Orabe; Joris Segonds, Herschel Jantjies; Ignacio Calles, Lucas Martin, Junior Tagi; Ewan Johnson, Lucas Paulos; Alexandre Fischer, Arthur Iturria (capt), Nika Lomidze.
Replacements: Facundo Bosch (Martin 51), Emosi Tumania (calles 57), Emerick Setiano (Tagi 51), Alvaro Garcia Iandolino, Baptiste Heguy (Iturria 46), Manex Ariceta (Lomidze 52), Guillaume Martocq (Tuilagi 49), Victor Hannoun (Orabe 72).
Leinster: Jimmy O'Brien; Tommy O'Brien, Garry Ringrose, Robbie Henshaw, Joshua Kenny; Harry Byrne, Jamison Gibson-Park; Jack Boyle, Rónan Kelleher, Tadhg Furlong; RG Snyman; James Ryan; Max Deegan, Josh van der Flier, Caelan Doris (capt).
Replacements: Dan Sheehan (Kelleher 46), Jerry Cahir (Boyle 70), Thomas Clarkson (Furlong 16), Joe McCarthy (Snyman 55), Scott Penny (Van der Flier 74), Luke McGrath (Jimmy O'Brien 66), Sam Prendergast (Tommy O'Brien 42), Rieko Ioane (Henshaw 34).
Referee: Luke Pearce (RFU)
