Schumer and Apatow were looking for somewhere to have a quiet drink after promoting their hit comedy Trainwreck in Dublin, Ireland, when they saw newlyweds enjoying drinks with friends and family.
And when they learned that Irish singer and songwriter Glen Hansard was there to sing a few songs they decided to join in the festivities.
Hansard joined Apatow and Schumer to lead the wedding crowd in a singalong of the famed Irish classic The Auld Triangle.
Schumer fumbled a few of the lyrics, but the newly married couple could not care less.
Schumer, arguably the hottest name in comedy, wrote the comedy Trainwreck, which has taken more than $101 million at the box office worldwide. The movie stars Schumer, Bill Haden and Tilda Swinton.
The comedy film was released in the UK on August 14 and is still to be released in many parts of Europe and in South America.