Longford

Will you have a car?

yes we will rent a car

With only 6 days at your disposal you're best advised to not try to do too much or cover too much ground lest your whole holiday will be spent behind the wheel. The natural urge for a first time visitor is to "see everything". Don't. Slow down and take you time.
Nice that you have relatives in Longford and Athlone, which are not that far from Dublin. Assuming that you'll be spending at least one night in each place, and another night or two in Dublin itself, that only leaves you a few days of discretionary time. There are several places within an hour or so of Dublin that might be of interest. North of the city is the Boyne Valley, the Hill of Tara, and Newgrange. Each is worth some of your time. To the south is the Old Military Road thru the Wicklow Mountains (just off the M50) down to the monastic settlement at Glendalough. With the freedom of a car you could plan to get there early, ie well before the tour buses start to arrive at 9:30. Could also do the short side trip over to Powerscourt, although you'll miss the glory of the gardens unless you plan to be there in the springtime.
Do allow for extra time to just meander - it's one of the great pleasures of driving in Ireland.
And ditch the car before venturing into Dublin - the traffic can be daunting even for experienced drivers, and having one in the city is just a hassle.
Safe travels.

Thank you so much for all the great info!!

I see on the map that between Longford and Athlone, there is Lough Ree. It looks like you could do some fun hiking and exploring around in that area - perhaps with newly-met family. I visited Ireland for the first time a year ago, and one of the many things I appreciated about it was how beautiful natural areas have hiking trails so they can be enjoyed, even if they're not the big famous places.