Firstly be sure which country Ireland or Germany is your country of residence. It is unlikely you will be challenged but just be sure you have some valid address ID for the country you deem your C of R.

The I/O rules is very confusing but I have simplified it as this for mobile passes:

At your choice you can freely travel on up to 2 of your travel days in your own country, These journeys can be in any way you would use your pass outside your own country. They do not need to include a border crossing in either direction, but obviously can if you wish. 

The intention is to facilitate your interrail experience by letting you start and finish your trip from home, but the implementation is much freer than that. You could use it to go to an airport and then resume your train journey from your arrival airport. Alternatively you could use these 2 days for return journeys from 2 trips where you flew out ( I did this recently with a flexi pass).

The only extra advice I would give is to understand the Travel Day rules - you can board any train scheduled to leave between 0000 and 2359 (CET) even if it is travelling after 2359. 

On a mobile pass no prob at all.

On a paper-the DB/German Schaffner may object-or not.

IF you live long enough in IE to get a residence permit-official, then use it and IE as ´home´country.