Both midfielder Richard Kennedy and new striker Tony Bird have recent experience of playing in the Eircom League.
Kennedy said: "They're a good strong side, don't give much away and have a lovely pitch.
"It's always great to have an idea of what you're up against and I'm looking forward to it."
Tony Bird meanwhile has played with both St Patrick's and Drogheda United.
Carmarthen boss Mark Jones said: "Like us Longford are a semi-professional side and are currently in mid-table.
"That will give us a very realistic chance of making it through to the second round."