Irregular has four syllables with stress on the second: ir-REG-u-lar. The Cambridge Dictionary transcription for British English confirms second-syllable stress, marked by ˈ after the first syllable. Check a dictionary with audio for the full American and British transcriptions.
How to shape it
Say the first two syllables with one smooth push: ir-REG. Let your tongue tip curl or bunch toward the /r/ without touching the roof of your mouth, then drop your jaw slightly for the stressed /ɛ/, as in red.
Glide into -u- by easing the lips into a small, rounded shape, the same start as the /j/ sound in yes. Keep it light; the stress stays on the second syllable, not here.
Finish the last syllable as -lar in American English, tongue tip lifted again for the final /r/ with lips slightly rounded. In British English the ending is /lə/, a soft luh with the tongue low and the /r/ left out.
Keep /r/ and /l/ apart in -lar: for /l/, the tongue tip presses the ridge behind your top teeth; for /r/, it pulls away and nothing touches. Say lar slowly a few times, then fold it back into the whole word.
Say it in a phrase
The schedule is irregular. Say it so the stress lands on REG, and the rest of the sentence stays light: the SCHED-ule is ir-REG-u-lar.
Contrast it beside its opposite: regular, irregular. Both keep second-syllable stress, so the only change is the prefix ir- added at the front, and the shared -ular ending should sound identical after the stressed syllable in each.
Try a question form: Is your heartbeat irregular? Lift your pitch slightly on irregular while keeping the stressed second syllable steady.
For British practice, end softly: ir-REG-u-luh, with no final /r/. For American practice, curl the tongue tip back: ir-REG-u-lar.
Use it about weather or verbs in a neutral phrase: an irregular pattern, then irregular verbs, letting the rhythm stay the same in both.