An irregular meter is one that doesn’t fit into duple or triple categories. They are sometimes called “odd meters” or “asymmetrical meters.”
Most people can’t. That’s why, when the time signature has a 5 in the numerator, conductors will often beat time in an asymmetrical two as 2+3 or 3+2. (In slower tempi, they may beat in five separate beats. That’s OK too.)
For time signatures with a 7 in the numerator, conductors will often beat in an asymmetrical three as 2+2+3 or 3+2+2 (or, in more unusual contexts, 2+3+2). It’s also common to beat in an asymmetrical two as 3+4 or 4+3. (And of course sometimes a conductor will conduct in seven too.)
The instructor will conduct and say two measures of time before beginning the dictation. Take careful note of which beat is the long beat. In this example, the instructor will be conducting in three: “one-and-two-and-three-and-a.” You can take note of this above the staff by writing a vertical line for a “two” and a triangle for “three.” That way you will remember which beat is longest.