Well,I had never thought about this possibility,but I find interesting to speak about it...I must say that Ennis die,or begun to die,the day he knew Jack was murdered.Although at the same time,he began somehow a "new life",since his "coming out" before Jack's parents was a kind of liberation from him,introducing himself without really hiding his love...But surely he didn't take much care of himself,as someone has said,mainly being a lonely man,for incapability or perhaps,a kind of self-punishment?.
Being as it's a movie full with methaphors,it can be one of them the wind, as an announcement that he's going to reunite himself with Jack at the end.The same than some phrases that have been remarked here,can be perfectly an announcement of an accident as a way to this reunion.Why not?Or simply,an heart attack,because Ennis-even if he wasn't an old man still...-has prematurely get old from this lack of care or for sufferings-anyhow,several years had passed,according to the s.s.-.Or ,I repite,because he didn't want,he needn't to keep on living...
P.S.:About a sink-pissing ghost...well,more than Ennis ghost,it would be Jack's ghost,who was funnier...