IR3535, short for Insect Repellent 3535, is a synthetic insect repellent originally developed in the 1980's by Merck, which holds a trademark on the name. Despite its imposing chemical name of Ethyl butylacetylaminopropionate aka Ethyl N-acetyl-N-butyl-ß-alaninate, IR3535 is just a synthetic amino acid which messes with an insect's sense of smell. It is structurally similar to the natural substance β-alanine - a beta amino acid which is, among other things, a component of pantothenic acid (vitamin B5). It has been used in Europe for over 30 years, where it has established an excellent safety record. The French Ministry of Health recommended it for for use by pregnant women and children during a recent Chikungunya outbreak, and it is one of the only four repellents recommended by the CDC for protection against mosquitoes carrying Zika Virus .
In over 3 decades of use, IR3535 has acquired an excellent safety record. An evaluation by the WHO  (World Health Organization) found only mild side effects including:
The same report stated that IR3535 is unlikely to pose any long-term risks in humans or (in normal use) any risks to the environment.
Studies have shown IR3535 to be effective against the Aedes mosquito species, known to carry West Nile Virus, Zika, Chikungunya and Yellow Fever. It is not as effective against Anopheles mosquitoes, and should not be relied upon in regions where Malaria is a concern. 
In repelling ticks, IR3535 is at its best. Studies have shown it to provide the longest lasting protection against deer ticks (Ixodes scapularis) . In higher concentrations, it is also effective against nymphs of the lonestar tick (Amblyomma americanum) , which carries Rocky Mountain Spotted Fever. It is also effective, though not the top performer, against the European castor bean tick (Ixodes ricinus.)
IR3535 offers similar protection against most species of mosquitoes and very effective at repelling deer ticks, without the odor, staining or skin irritation associated with DEET. A 2009 study in Queensland Australia  comparing products with DEET and IR3535 found a 35% concentration of Deet more effective and longer lasting than Avon Bug Gard, with 7.5% IR3535. However it should be noted that this study pitted a product with a fairly strong DEET formulation against one with the lowest commercial concentration of IR3535.
Deet is more effective against the Anopheles species of mosquitoes, which carry malaria, encephalitis and dengue fever. This is primarily a concern in tropical regions and sub-Saharan Africa.
A small 2007 study  concluded that picaridin (Bayrepel) and IR3535 were similarly effective against Aedes aegypti- a mosquito species known to transmit Zika Virus, West Nile, Dengue Fever, Chikungunya and Yellow Fever. These mosquitoes are found primarly in the south and southwestern United States, but their habitat also includes much of the Eastern part of the country - extending as far north as Connecticut and west through southern Indiana.
Like other repellents, IR3535 is applied directly to the skin. Always follow directions on the product label. An application typically lasts 4-8 hours. Avoid applying it near your eyes or mouth, on wounds or irritated skin. and never spray IR3535 (or any repellent) at your face. Although not believed to be toxic if ingested, you should still keep it away from food and food preparation areas, and not apply to childrens' hands.
 - http://www.cdc.gov/zika/prevention/index.html
 - http://www.who.int/whopes/quality/en/IR3535_eval_april_2006.pdf
 - https://www.ncbi.nlm.nih.gov/pubmed/26900115
 - https://www.ncbi.nlm.nih.gov/pubmed/24201040
 - https://www.ncbi.nlm.nih.gov/pubmed/20695288
 - https://www.ncbi.nlm.nih.gov/pubmed/20099600
 - https://www.ncbi.nlm.nih.gov/pubmed/17252270