Beinn Alligin means a Mountain of Beauty in Gaelic language and I must agree with that. It consists of two tops - Munros (the mountains of more than 3000 feet in height).
Usually the best time to visit Scotland is from mid April till the end of May but you can be lucky and experience good weather all year round. April is great with the patches of snow, brown grass and blue sky contrast whereas in August you can experience nice green/pink colors. Can be crowded during weekends with good weather but it is still memorable experience.