While none of the facts in the existing writeup are wrong, the quantity of information here on one of the most unique, unexpected, and successful classic rock influences of all time seems quite anemic. So let's start from the beginning, with a black Irishman named Phil Lynott.
Born in 1951 in Dublin, Ireland to parents of Irish and Brazilian descent, vocalist bassist and songwriter Philip Parris Lynott founded Thin Lizzy in 1969 with his childhood and lifelong friend, drummer Brian Downey. For having been one of the greatest guitar bands of all time, it is perhaps ironic or perhaps quite appropriate that the core of the band and the only two consistent members throughout the band's history would be the two men who made the rhythm section. They quickly recruited Eric Bell as the band's original lead guitarist, who had made a name for himself in a brief stint with Van Morrison's band Them. The original lineup also featured organist Eric Wrixon, who reportedly did not fit in well with the sound of the band at all, and by February of 1970 Thin Lizzy had dissolved into a trio. The name of the band was first suggested by Bell, who took the name from a robot character out of a U.K. children's comic book called The Dandy. The name "Tin Lizzie," the band's etymologically correct pronunciation, was mockingly modified to "Thin Lizzy" to account for the Irish habit of dropping the h in such words. The band found the wordplay amusing, and the name stuck. Some posters of the band's early gigs in the beginning of the 1970s would mispromote the band as "Tin Lizzy" or "Tin Lizzie."
Though the year of 1970 the band garnered a reputation as a strong live act in North Ireland (not to be confused with Northern Ireland). Decca Records sent an A&R agent from England to investigate, who then signed Thin Lizzy to the label in November of 1970. The band relocated to London to play gigs in 1971, but their live debut at the Speakeasy Club was underwhelming and unsuccessful. The reasons as to why the band was not so received in London are mostly speculative, but I would reasonably guess the obvious - London is much less of a rowdy, spilling drunkard fighter's town than Dublin, and was less prepared to receive the level of heaviness Thin Lizzy was putting on the table.
In any case the band would continue to struggle as they filled their contractual obligation to Decca, releasing their debut Thin Lizzy in April of 1971, and Tales from a Blue Orphanage in March of 1972. Both albums failed to chart. Things would turn around towards the end of 1972, as the band embarked on a tour of the U.K. with Slade. Around this time, Decca decided to release the band's recording of traditional Irish folk song "Whiskey in the Jar" as a single without consulting the band. The band was verbally upset by Decca's decision, feeling that the song did not well-represent their sound or their image. The single became a surprise hit however, peaking at #6 in the U.K. in February of 1973, ultimately becoming their highest charting single in the U.K., as well as hitting #1 in Ireland.
In spite of "Whiskey"'s success, the band disowned the song and chose not to include it on their September 1973 release Vagabonds of the Western World, an album in which the band clearly tried to distance themselves from the traditional, acoustic guitar riff driven Irish folk-rock by making their stance as a flashy and heavy rock outfit. The album was another commercial failure, and the ensuing frustration and disillusionment with the music industry would cause Bell to leave the band in January of 1974 under the grounds of exhaustion to stay home in Ireland.
The remainder of 1974 brought about many changes for the band, beginning with some lineup shuffling. For a brief 4-month interim from January to May, ex-Skid Row guitarist Gary Moore would see his first of several incarnations as a member of Thin Lizzy. In May, after having scheduled a tour of Germany under contract, the band temporarily brought in guitarists Andy Gee of Steve Ellis' band and John Cann of Bullitt to secure their touring potential. Shortly thereafter in June full-time guitarists Brian Robertson of Glasgow and Scott Gorham of Santa Monica, California signed onto the band. This would complete Thin Lizzy's classic and most famous lineup, and mark the origin of the band's trademark dual lead guitar sound.
In August of 1974 the band signed to Phonogram's progressive rock label, Vertigo to much excitement. By the end of the year, the band had released their Vertigo debut Nightlife to a greater reception of yawning than applause. Next year's September 1975 release Fighting would be the band's awakening point, finally finding comfort in their own skin. It would also be the band's first album chart in the U.K., hitting #60. Then in July of 1976 the band finally had their long awaited breakthrough - the appropriately named Jailbreak. The album hit #10 in the U.K. and stayed on the charts for 50 weeks. The album was a trans-Atlantic hit as well - it reached #18 in the U.S. charts and earned a gold disk. The lead single from the album "The Boys Are Back in Town" reached #8 in the U.K. and #12 in the U.S., and survives (at least in the United States) as the band's most popular song. In August of 1976 the album's eponymous track peaked at #31 in the U.K., and in October "Cowboy Song" became the second and last Thin Lizzy single to chart in the U.S., landing at #77.
By this point the band had arrived in their golden age. They would continue to land a litany of highly successful albums and singles, climaxing with the infamous 1978 double live album Live and Dangerous, highly and commonly regarded as one of the greatest live albums of all time. Though the album faced some controversy both at its release and through the annals of music history for its in-studio manipulation and production touch-ups, it hit #2 in the U.K. in June of 1978 and would stay on the charts for 62 weeks. Perhaps an even rarer feat, the live album produced a top 20 single - a medley of Bob Seger's "Rosalie" (originally appearing on Fighting) and "Cowgirl's Song," essentially just riffs and variations on the bridge of "Cowboy Song."
As these things tend to pass for the rock 'n roll lifestyle, the wheels began to come off shortly after the band's greatest successes of the mid-70s. I would contest that the beginning of the end came in August of 1978, when Brian Robertson quit the band(again, after an injury in 1977). Between 1979 and 1981 Thin Lizzy would go through four different guitarists to play alongside Scott Gorham, beginning with another Gary Moore period. Moore was fired by Thin Lizzy's management during a tour of the U.S. in July of 1979, to be temporarily replaced by future Ultravox guitarist Midge Ure, then by Pink Floyd's live full band guitarist Snowy White, who will leave in December of 1978 for a solo career after the band releases their worst record, the trite and stagnant Renegade. He is to be replaced by former Tygers of Pan Tang guitarist John Sykes for the short remainder of Thin Lizzy's "original" existence, though Sykes would have much to do with the band's second coming.
In the midst of these lineup changes, Lynott would come to feel as jaded with the group as Bell once did. In the early 1980s, Lynott married his wife Caroline Crowther, and made a small attempt to piece together a solo career. By the time Thin Lizzy recorded their first album with Sykes and last album with Lynott in May of 1983 called Thunder and Lightning, Lynott seemed ready to hang it up. He officially split the band in August of 1983, saying he felt the band had become predictable and directionless. What would follow in Lynott's final years would be more unsuccessful attempts at a solo career, the attempt to form a band named Grand Slam with Downey which would fail to be signed, and excessive drug abuse which would eventually lead to his death. He suffered a heroin overdose on Christmas Day, 1985, and would slip into a weeklong coma. He died on January 4, 1986 of liver failure, kidney failure, heart failure, and complications from pneumonia.
In 1996 John Sykes reunited Thin Lizzy with Gorham and Downey as a tribute band. They are still active today, in some capacity, although Sykes himself left the project in 2009, handing the reigns over to Gorham to spearhead the group. They have never recorded new material under the name Thin Lizzy, and have publically stated no plans to do so, out of respect to Lynott and his legacy. The touring/tribute band released new material under the name Black Star Riders in May of 2013 with their debut All Hell Breaks Loose. The band still tours as Thin Lizzy, and essentially functions as an homage.
Thin Lizzy, Decca, 1971
Shades of a Blue Orphanage, Decca, 1972
Vagabonds of the Western World, Decca, 1973
Nightlife, Vertigo/Mercury (U.K./U.S.), 1974
Fighting, Vertigo/Mercury, 1975
Jailbreak, Vertigo/Mercury, 1976
Johnny the Fox, Vertigo/Mercury, 1976
Bad Reputation, Vertigo/Mercury, 1977
Live and Dangerous, Vertigo/Mercury/Warner Bros. (U.K.; Canada; U.S.), 1978
The Continuing Saga of the Ageing Orphans, Decca, 1979
Black Rose: A Rock Legend, Vertigo/Mercury/Warner Bros., 1979
Chinatown, Vertigo/Mercury/Warner Bros., 1980
The Adventures of Thin Lizzy, Vertigo, 1981
Renegade, Vertigo/Mercury/Warner Bros., 1981
Thunder and Lightning, Vertigo/Mercury/Warner Bros., 1983
Life: Live, Vertigo/Mercury/Warner Bros., 1983
Dedication, Vertigo/Universal, 1991
Black Star Riders
"Rock Movers & Shakers, Revised and Enlarged Edition", Dafydd Rees and Luke Crampton
"The Rolling Stone Encyclopedia of Rock & Roll, 3 ed.", various authors
Liner notes from Dedication, entry by Neil Jeffries of Kerrang!, used sparingly. No copypasta. Never ever.