Indeed you have two options and either way you can claim the full expense incurred.
So, if you buy a rail ticket you claim the expense back from your company. Obviously it needs to be properly recorded and accounted for in your expense claims.
If you use your own car you are entitled to claim a mileage allowance. In addition, since you are away from what (I presume) is your normal place you are entitled to claim (from your company) a subsistence allowance.
Have a look at the links below and also have a word with your accountant to make sure you properly apply and record the allowances
http://www.revenue.ie/en/tax/it/leaflets/it51.html
http://www.revenue.ie/en/tax/it/leaflets/it54.html
Regards,
RUDOLF289