8:47 is a bit better than what is there currently but I agree that it needs to be after 9pm.
You could achieve a later service by finishing the 17:17 at Maynooth and returning it to the city rather than leaving running almost empty to Longford as is the case currently. Presumably this new service would return to Dublin empty after getting to Mullingar.
You could also achieve mostly the same result by allowing commuters use passes on the 8PM and 11PM Bus Eireann expressway services and then get some good data as to demand. But of course this is physically impossible because these are commercial services and joined up public transport is evil.
Personally I've most given up on the train service on this line after nearly 15 years commuting. I'm only travelling occasionally now, but unless I'm going to have to travel at both rush hours, it's generally far quicker to access the city centre by car and not much more expensive. Leaving at the same time as I'd catch the first train from Longford gets me into the city centre almost an hour before the train which is just nuts. The heart says train - the head says car.