In Synge's, Riders to the Sea, what does Maurya mean, "No man at all can be living forever, and we must be satisfied"?

It is obvious from the beginning that this play is a very Roman Catholic story of a mother's religious maturation.  Unfortunately, this maturation takes place through tragedy:  the death of her sons (... of ALL her sons).  The final product of all the learning in Riders to the Sea ends with Maurya speaking the words you mention.

As described in the above answer, there is a grand irony in Maurya's words.  As she waits for Michael to return from the sea, she moans for Bartley (who hasn't even left the home yet and is working on a rope to bridle a horse).  For the majority of the story, in fact for 99.9% of the play Riders to the Sea, Maurya has not yet reached spiritual maturity.  She is still longing for the life of all of her sons.  She wails for the ones that have already passed.  She waits for the one on the sea to return home safely.  She urges the one still at home not to leave the home, ... even if only for a horse fair.

There is quite a bit of foreshadowing beforehand to show that in Riders to the Sea Maurya will lose all of her sons.  Most specifically, there is the vision she has of herself unable to give her sons a blessing while Bartley rides a red horse and Michael a grey horse.  Sure enough, Maurya's daughters have already discovered that Michael has drowned, and the news is about to arrive that Bartley has drowned as well.  In fact, Maurya finds out by seeing Bartley's wet, dead body. 

To get back to your question, though, the last statement by Maurya is one of spiritual maturity:  a realization that no human, no man, and no SON (not even sons of hers) would be able to live forever on this earth.  The only hope is of the afterlife, as is indicated by the religiosity throughout the play and especially with all of the Signs of the Cross done with the entrance of Bartley's body.  Why must we be satisfied?  Because it is GOD that has given us life, ... has given Maurya's sons life, ... and it is GOD that will receive them happily after they all die.  The fact that Maurya's sons die before her should have no consequence.  Their life now will be in heaven in perfect bliss.  One must understand, though, that Maurya's satisfaction is NOT with death, but is in GOD.  Period.  Hence, the tragic, one-act play ends with the following:

No man at all can be living forever, and we must be satisfied.