An absolute wordsmith, 'a pair of brown eyes' is as close to a perfect song. Novels of thousands of pages haven't told a story as deeply as that song. His version of the Band played waltzing Matilda is one of the most moving songs I've ever heard - and amazingly wrote fairytale of New York as a bet, when Elvis Costello bet him he couldn't write a Christmas song.
Wordsmith is correct and a man with abundant talent. There's a great fairly recent documentary about his rocky life called Crock of Gold.
Regarding his aptitude for song writing there's a interview with Nick Cave where he covers a Pogues song... Cave describes spending time with Shane McGowan and him having a pile of songs in a mound on the floor. Picking them up he sees there are beautiful songs just lying there most destined for oblivion. A prolific song writer to say the least. You'd wonder how many gems were lost to his irreverent nature.
I saw Shane and the Popes (and a great New York band Black 47) open for Elvis Costello in NYC many years ago. Shane walks on stage and immediately hocks a loogie in the face of a random audience member standing up front. ‘Twas a hell of a show.