Irritable baby syndrome is described as a baby who is often unsettled and cries for no apparent reason.

Babies may become irritable if they are hungry, tired or have a wet or dirty nappy.
Babies who have experienced a hard and fast delivery, a prolonged birth or a complicated birth with the use of vacvumm extraction or forceps may be irritable due to muscular aches and pain.

It has been estimated that 73% of babies may suffer with musculoskeletal injuries following birth. These musculoskeletal injuries may significantly affect an infant’s quality of life. Babies with musculoskeletal injuries are more likely to cry excessively (1).

In a study of 316 infants suffering from excessive crying, 94% showed dramatic improvement following Chiropractic treatment(2).
Another study compared Chiropractic treatment and the drug Dimethicone. It showed that Chiropractic was significantly better in reducing crying time than the drug Dimethicone (3).

If you have an irritable baby then having your baby checked for a chiropractor may identify musculoskeletal problems. Having these issues corrected could make your baby more settled.
Just like if you have a sore neck and cannot get comfortable. Once your neck is better then you no longer feel the discomfort and are much happier.
REFERENCES
(1). Joyce, M.,Fontana, M., Jernlas, K.,Olofsson, H & Verwijst. I. 2013. Risks and rewards of early musculoskeletal assessment. Journal of Health Visiting. Vol 1: Issue 10: 590-596
(2). Klougart. N, Jacobsen. J, Niels, K (1989). Infantile Colic Treated By Chiropractors: A Prospective Study of 316 Cases. JMPT 12:4: 281-288.
(3). Jesper. M, Wilberg. M, Nordsteen. J and Nilsson.N. (1999). The short term effect of spinal manipulation in the treatment of infantile colic: A randomized controlled clinical trial with a blind observer. JMPT. 2