The person on the train should be in a position to provide assistance to people who need it at stations if they aren't hiding in the cab.
As for walk-up tickets - this would have to apply only to stations that have ticket machines which should sort that problem out. A large portion of the people who would have difficulties with ticket machines would be on free-travel passes anyway. In terms of pricing, why should a ticket machine be any different to buying from a person - I wouldn't expect to get an on-line discount for using a ticket machine. It's a walk-up ticket just the same.
It might be worth either manning stations or allowing on-board ticket purchases on Saturday mornings when there would tend to be a lot more walk-up business.
To me it's a no-brainer when resources are limited to move staff from stations to trains. Hopefully, Irish Rail's customers spend more time on trains than in stations, so it makes perfect sense to ensure that there is somebody there to provide assistance on trains. In an ideal world, we could go back to the 1940s when rural stations had a stationmaster, a ticket-clerk and two porters and every intercity train had a driver, a stoker, a cook, a barman, a steward, a guard and a ticket checker but personally, I can't afford 100 euro return tickets from Edgeworthstown to Dublin.