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I've seen some warnings regarding IR sensors and transmitters such as the 4th panel below for the Nintendo 3DS:
The Operations Manual says,
Do not get too close to the infrared transceiver
Looking directly into the infrared transceiver can lead to impaired vision and other problems.
But if IR is lower frequency than visible light how can it be bad for your eyes? What are the possible adverse effects of looking straight into an IR transmiter?
Take a look at the spectrum for sunlight. About half of the energy emitted by the sun is infrared (see citation 6). Some of this is attenuated by the atmosphere, but approx. 30% of the 1000 watts of sunlight power that reach sea level are in the infrared spectrum.
IRDA systems use IR leds such as this one. These have emission spectra in the 900-1000nm range (completely covered in the ir emission spectra of sunlight), and output powers <<1 Watt. (The one pictured above has a max power of 0.075 watts).
Therefore, going outside on a bright day will deliver >3000 times more infrared energy in the same spectrum to your eyes than an IRDA port.
Quick answer: it's harmless. Maximum radiation dose is 20 times smaller than the safety limit.
Long version: Well it all depends on several factors:
Calculating it is not an easy task, even for the professional (you can find ISO standard, which is under code 10110 if I'm not mistaken, however you'd need to pay an access fee).
Generally speaking, you could assume that typical devices with output power under 20mW are safe to look at for several seconds if the beam diameter is above 2mm. It might partially blind you (glare effect for visible spectrum and NIR) and maybe cause minor, insignificant damage. Below is how well are different wavelengths (IR is above 950nm) detected by human eye:
EDIT:
http://www.dtic.mil/dtic/tr/fulltext/u2/663246.pdf
You can find necessary standards here (most importantly ISO 11254-2): https://www.iso.org/obp/ui/#iso:std:iso:10110:-17:ed-1:v1:en
Simplified calculations Assuming classical IrDA diode like: http://www.excelitas.com/downloads/DTS_CR50IRDA.pdf
and typical eye (pupil and focal length) with ideal optical transmission case (no power loss in air and eyeball) as well as ideal transformation:
[ https://en.wikipedia.org/wiki/Geometrical_optics ]
We have a set of variables: - pupil diameter: 7mm (average) - eye lens focal length: 18mm - eye diameter: 24mm - lighting angle: 160 deg - IR light power: 20mW - wavelength: 870nm
Assuming that you touch the IrDA port to your eye, the distance between light source and eye would be 5mm, then, using the above equation with given data, the light limited by 7mm pupil will focus 7 mm behind lens, so basic geometric analysis gives as spot diameter on retina equal to (24-7)7/7 = 17mm. At 5mm distance, diode irradiates total field of pi*(5mm / cos(160/2 deg )^2=830pi mm^2 but only pi*(7mm/2)^2 = pi*12.5 mm^2 is transmitted through eye pupil.
Concluding: your retina would be exposed to 1,5% of 20mW power on 900 mm^2 surface of retina which is 3 * 10^(-4) W over 9 cm^2 = 3,3 * 10^(-5) W/cm^2 where the laser standard (more restrictive) for wavelength of 900nm is 7 * 10^(-4), so it is 20 times smaller than the safety limit.
[ according to "Safety with Lasers and Other Optical Sources: A Comprehensive Handbook" by D. H. Sliney, J. Mellerio ]
Here is a brief description of why IR can be dangerous.
Despite not being visible, IR rays can still pass through the anterior structures of the eye and reach the retina. Since you are unable to detect IR, there will not be any blink or aversion reflex to protect your eyes from damage.
Getting too close and staring into an IR transmitter is effectively the same as getting too close and staring into a flashlight - the mechanism of injury is the same.
The actual standards for IR protection can be found here.
