Here is the example from the documentation:
<html>
<body>
<form id="loginForm">
<input name="username" type="text" />
<input name="password" type="password" />
<input name="continue" type="submit" value="Login" />
<input name="continue" type="button" value="Clear" />
</form>
</body>
<html>
login_form = driver.find_element_by_xpath("/html/body/form[1]")
login_form = driver.find_element_by_xpath("//form[1]")
login_form = driver.find_element_by_xpath("//form[@id='loginForm']")
input[1] is an array, starting at 1, so in your case, it should looks like this(Corrected, following comment):
button = driver.find_element_by_xpath("//form[@class='smallfont']/input[@value='Ir'][@type='submit'][2]")
button.click()
From:
http://selenium-python.readthedocs.io/locating-elements.html