They are probably using the 29K that normally sits in Longford all weekend and one of the Longford drivers. It isn't a lot more expensive to run in service out to Longford than it would be to run empty back into town after an extra late-night Maynooth service. Might even be cheaper to run to Longford given that the only remaining manned level crossings are between Maynooth and Dubin.
For some reason Irish Rail have always been very reluctant to put concert specials on the Sligo line but this is silly now that the line is automated. On the lack of advertisement and notice, maybe they're trying to prove that there is no need to run concert specials by setting this up to fail. Perhaps the NTA is putting them under pressure to run a regular later service on the line given how much money has been invested in automation and this would be a good data point showing that even when there is a concert on the service is unviable.
Not that I'm cynical about Irish Rail or anything. In reality, it's more likely they just figured out that it costs about the same to go on to Longford than to run empty to town, and a Longford driver volunteered for a nice wedge of overtime.