Hoping that you will be earning enough to make such a commute sensible, going the Taxsaver route would be important, as you can save up to about 52% of the price of the ticket. However, your employer will need to take part in the scheme. You can get through tickets between any two points on the Irish Rail / NIR networks and this is valid for all journeys within those zones, e.g. you will be able to use it on a trip from Lansdowne to Pearse. Check the limits for Newry, it might be that you can travel south of Lansdowne for the same price
Weekly tickets should be available in the ticket office. Monthly tickets might be available in the ticket office. You will need to check the photo ID requirements. Annual tickets are likely to only be available on a personalised Leap Card.
All tickets can be bought in advance, but make sure you specify (and check) the date.
Depending on currency fluctuations, there may be a price difference on where you (your employer) buys.
Are there ticket gates at Newry where you need to validate a ticket? It is likely that you will be able to use the Leap Card as a 'flash pass' that you show the ticket checker. There is a validator for transfer passengers on platform 5 in Connolly Station.
AIB have a private platform access at Lansdowne. I'm not sure if others can use it.
You do not need two tickets / passes. This will not be cheaper. That said there are some funny things that happen with single / return tickets, where two tickets can be cheaper.
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