I set a reminder in my phone to check out this cairn at sunset around Bealtaine as Martin Brennan suggested it has an early May alignment. Well, I since switched phones so it was only while looking for a drawing of something else (the stone at Killin in Co. Louth, which he seems to have confused with Carrickrobin) that it hit me. As luck would have it, the weather then wasn't great but tonight it was perfect.
The sunset does indeed stream down the left hand side of the passage (as looking out) and forms a rectangle of light on right side of the backstone which slowly decreases in size as the sun reaches the horizon. It may have even entered the now-destroyed right hand chamber before dipping below the horizon.