Over the years different Musicians have been "special guests" with the Dubliners, or filled for regular members on a short term basis. Below are the musicians … of which I am aware.
Mary Jordan: During the approximate period of 1962 to 1965, Mary Jordan would sit in with The Dubliners. "… the ingredients of a group that was not yet a group, more a loose-knit company which came together at regular intervals. At this time it was quite usual for Peggy Jordan's daughter Mary to sit in on the sessions playing spoons." 
Anne Mulqueen: "For a short while too the casual assembly was implemented by Anne Mulqueen, a singer from Limerick and a friend of Barney's."  This would seem to be around 1962 or 1963.
John Reavey: After Luke Kelly left The Dubliners in 1964, John Reavey performed with the band. Apparently, with and without Bobby Lynch and John Sheahan. 
Danny Doyle: Ronnie Drew told me that Danny Doyle stood in with The Dubliners for an ill Jim McCann. Unfortunately, I don't have any further details …
Ronnie Drew: Ronnie, himself, has stood in when he wasn't actually in the group at the time. In 1978, The Dubliners took part in Siamsa Cois Laoi, Cork, filmed at Páirc Uí Chaoimh, Co.Cork.This performance featured the unique line up of Ronnie, Luke Kelly, John Sheahan and Jim McCann. Apparently, Barney was in Europe shooting The Green Linnet — a documentary TV series for RTÉ, with Tony McMahon. I assume, Ronnie was filling in for Barney. This is one of the few, if not only appearance (that I know of) with Ronnie, Luke and Jim McCann together.
Ronnie also made guest appearances with The Dubliners, at least twice, after he left the band for the 2nd time. In June 2004, Ronnie joined the band to sing "The Irish Rover" on RTÉ. I am fairly certain this was a Phil Coulter TV special. The second occasion was after Patsy Watchorn joined the band, on December 12, 2005 at the Submarine Bar in Dublin. Apparently, Sean Cannon was unable to make the Dublin gig, and Ronnie stepped in.
Eamonn Campbell: Prior to becoming a group member Eamonn played with The Dubliners in the 1980's, perhaps even on occasion in the 1970's. Examples are: 21 Years On; Visions of Ireland & The Late Late Show Tribute
Jim McCann: In addition to his tenure as a full-time member and participation in the 40th Anniversary Celebration tour; McCann appeared with The Dubliners, in 1983, on the Visions of Ireland VHS. Jim also filled in on guitar for Eamonn Campbell in April 2005 for some dates in England, after throat cancer ended his singing career.
Paddy Reilly: Before taking Ronnie Drew's spot in The Dubliners, Paddy Reilly too appeared with the band on Visions of Ireland. Additionally, Reilly appeared with the band in 1984 on RTE's the Late, Late Show special marking The Clancy Brothers and Tommy Makem reunion. Paddy also mentions in an interview that he filled in with the band on different occasions (over the years) before joining the band, but I don't have any specific details of any of these appearances.
Nigel Warren-Green: Classical cellist, turn film produced, toured with The Dubliners, in the early 1980's.
Michael Howard: Classical Guitarist, Michael Howard, who collaborated with John Sheahan on the album, In Our Own Time, made guest appearances with The Dubliners in the 1980's. Again, I don't know how often, but he played with the band at concert I saw in the late 1980's.
Michael Howard also toured with The Dubliners for 3 months in 2006 (September/November), filling in for an ailing Eamonn Campbell.
Bobby Kelly: Bobby Kelly, a Dublin guitar and banjo player, performed with The Dubliners. I don't know how often, but there is (was) a picture hanging in O'Donoghue's Pub, Dublin of Bobby playing with band. I met Bobby in 1986, when he was accompanying Ronnie Drew at a gig at the Wexford Inn, Dublin. Bobby was a very good friend Ronnie's, and backed him up on solo gigs on many occasions. On a side note, Bobby is a very nice guy, and I hope he's doing well.
Gerry O'Connor: In September 2005, Gerry O'Connor filled in for Barney McKenna in Vienna. Gerry is one of Ireland's most respected musicians and Barney personal choice to take his spot with The Dubliners. After the death of Barney McKenna, Gerry filled in for the bulk of the remaining 50th Anniversary tour dates in 2012.
Al O'Donnell: Al O'Donnell took Patsy Watchorn's spot during The Dubliners 2010 fall/winter German tour. Al is (another) very well-respected musician on the Irish folk scene — since the early 1970's. Al also accompanied Luke Kelly (on guitar) in 1979 on RTE's The Humours of Donnybrook for Luke's stirring rendition of "Raglan Road". This clip was included on the 2005 DVD (& CD) Luke - The Performer.
Chris Kavanagh: Chris Kavanagh filled in for Patsy Watchorn on the second part of The Dubliners 2011 German tour (November 23-December 3). Chris also appears on the DVD, 50 Years - Celebration Concert in Dublin
The Dubliners Scrapbook Clippings …
INTO THE BREACH GOES THE RONNIE DREW GROUP
THE withdrawal of "Bloomsday" after a five-week run at the Gate Theatre has not caused him any disappointment, claims John Ryan. Although he had at first visualised a seven-week run, taking him right up to the end of his long tenure of the Gate, John acknowledges that a revival is extremely fortunate in getting the sort of run "Bloomsday" has had.
He adds: "In any case, I would have had difficulty in keeping the cast together, since no less than six of them were looking for permission to leave — most of them had been signed up for "Finian's Rainbow" at the Gaiety. "I've never known a time in Dublin when actors were more in demand."
Into the breach next Monday he throws the Ronnie Drew Group in a Hootenany type show called "The Dubliners." This title pays due homage to the recently released long-playing record, on the Transatlantic label, by the Ronnie Drew Ballad combination.
According to trade sources, this long player is showing all the signs of setting new records for a ballad disc. Recorded in London three months ago, it was the first part of a two-year contract, which the group recently negotiated.
The work could hardly be described as arduous, since the contract calls for a maximum of fifteen hours' recording for each of the two years.
"Anything over and above that calls for a new contract," said Ronnie Drew with quiet satisfaction as he lowered a pint in the Merrion Row temple of the balladeers' art.
With him he had John Reavy, a Drogheda-born folk singer, who will be appearing in Dublin for the first time next week. Brought up in the north-east of England, Reavy has a repertoire of Geordie and Scottish border songs to draw on for "The Dubliners."
But, surprisingly, he admitted to knowing few of the emigrant and work songs of the Irish labour force in England. "I heard a lot of Irish songs from the emigrants in Newcastle, and I'll be singing these at the Gate," he promises.
The folk and ballad singing the Dubliners, pictured with Mary Jordan, who accompanies the on them on spoons, backstage at the City Theatre, Limerick, where they are playing all this week.
Top (from left): Bobby Lynch and John Sheahan. Centre: Barney McKenna and John Reavey. Bottom: Ronnie Drew and Ciaran Bourke
The above clippings come from the Dubliners Scrapbook.