Longford-Dublin family ticket prices

[TheQ47]

Hi, new user here, semi-regular train user, a couple of times a month.

I was looking at bringing my family from Ballymote to Dublin Connolly on a day return this Sunday, 5th June (going to the Bloom Festival in the Phoenix Park). Looking online, it would cost me €132 (i.e., €44 per adult, and €22 for each of 2 children).

I tried to ring the station in Sligo, and the guy on the phone told me it would cost me the same by buying in the station.

Someone told me that if I were to drive to Longford, park there, and buy my day return from there, it would only cost €15 per adult, and €7.50 per child, a total cost of €45. Is this correct? I can't find any sign of the fare from Longford-Dublin on the irishrail website, so I'm a bit dubious that it would be this low.

Does anyone know if these prices are correct? I can't book online for a ticket from Longford (why is this?), so I can't check the price there. Because we're travelling as a family, I'd like to be able to book my seats in advance, to ensure that the 5 of us could travel together.

Is there a cost for parking in Longford? I used to do this regularly before the "new" Sligo-Dublin service, so I don't know if this has changed now.

Any help would be appreciated.