What kind of train is used from Holyhead to Birmingham Int?

Does anyone know what kind of train is used from Holyhead to Birmingham International? I am advising someone on taking the ferry from Dublin. She is worried she needs a seat reservation. I am sure she doesn't. The train will be Arriva trains Cymru/Wales.

(The combined price for the ferry and the train to south of England is about £35..)

Why doesn't she just buy the ticket in advance (the cheapest method) and take care of the seat reservation that way?
http://www.arrivatrainswales.co.uk/Home.aspx

kaitoku wrote:Why doesn't she just buy the ticket in advance (the cheapest method) and take care of the seat reservation that way?
http://www.arrivatrainswales.co.uk/Home.aspx

I assume it's a DMU, as most of Arriva Trains Wales are DMUs. When I travelled from Birmingham to Aberystwyth in one last July we had seat reservations but there was clearly no need for one. She says the Dublin office she tried (at Busarus) wouldn't do seat reservations, but I am pretty sure the Connolly ticket office will do it.
This is the relevant information.
http://www.seat61.com/Ireland.htm
Note the very low price even for same day of travel - possibly the best bargain in the British Isles. My wife and I may make this trip next year.