How to unite units?

You send them to the same location. Easy as pie.

Sometimes clicking the "march" action makes it easier to combine units.

If you have a mouse holding down both buttons creates a kind of net which you can encircle the units of choice with, when you order them to a location they combine.

Never knew that, thanks for the neat trick.

I found it out by accident.

British army, with power over 2k, attacking Ireland is normal?)

Is it not normal?

Not mentioned above:

Click on a unit/stack, press Ctrl (hold), press on all the units/stacks one wishes to combine, click on "March" or "Attack", select destination. This procedure allows one to *not* select certain nearby units/stacks.

If one would like all these units to arrive at the same time (ie. for a simultaneous attack), follow the above; then press "Delay". This will delay each unit/stack the appropriate amount of time for a simultaneous arrival time.