On occasion, I hear things such as 是非、いらしてください and ここへいらしたんですか. They seem to be politer forms of 是非、来てください and ここへ来たんですか. But searching finds no verb いらす or いらしる, and so a thought came to my mind: could they be colloquial forms of いらっしゃって and いらっしゃった? Less colloquially, would my examples be 是非、いらっしゃってください and ここへいらっしゃったのですか? Is my suspicion close to the truth?
It's ambiguous whether いらして would be a form of いらしる or いらす, but neither verb exists in the standard language.
いらす isn't listed in dictionaries as a word because it's not a separate verb with a full range of forms. It would be more accurate to say that いらし is a reduced form of いらっしゃっ, the 音便形 of いらっしゃる. (The 音便形 is the altered form of the 連用形 that appears before t-morphemes such as て and た.)
This reduction is most common before て. If we search the Google Japanese Web N-gram corpus (2007), we find the following numbers:
いらっしゃって 355269 いらして 1620011
As you can see, this reduction is extremely common! We can verify that いらして is the most common form by checking other corpora. In the Balanced Corpus of Contemporary Written Japanese, we find a similar ratio, with 285 and 90 results respectively (after removing false positives with the form いらいらして).
Although いらし is most common before て, it appears before other t-morphemes as well. I'll go back to the Google Japanese Web N-gram corpus, since it's significantly larger:
いらっしゃった 554636 いらした 597706 いらっしゃったら 239239 いらしたら 182970 いらっしゃったり 16167 いらしたり 8085
If いらしる were actually a derived verb that needed to be listed in dictionaries on its own, we would expect to find examples of it as well, but we don't find any at all:
いらっしゃる 4533100 いらしる 0
We do find いらす, but it's not especially common, and prescriptive sources consider it a mistake (it's listed as 誤用 in 明鏡国語辞典):
Samuel Martin describes several reduced forms of the 音便形, including いらし, in his 1975 Reference Grammar of Japanese on page 347:
Both men and women readily use several variant forms for irassyát-, so that in addition to irassyátta you will hear irássitta, irásitta, irássita, and irásita. These variants no doubt were gradually developed to simplify the articulation: the low vowel a is raised to the high vowels u and i so as to become unvoiced between the voiceless consonants; the double ss and tt are reduced to shorten the word.
And it's possible you may hear other reductions besides いらし. But the other forms he lists are all quite unusual by comparison:
いらして 1620011 いらっしゃって 355269 いらっして 3351 いらっしって 344 いらしって 895 いらした 597706 いらっしゃった 554636 いらっした 1586 いらしった 1448 いらっしった 390 いらっしゃったら 239239 いらしたら 182970 いらっしたら 226 いらしったら 152 いらっしったら 67 いらっしゃったり 16167 いらしたり 8085 いらっしったり 0 いらしったり 0 いらっしたり 0
These other reduced forms are listed in other sources as well, including 日本国語大辞典.
Reduced forms of words in general are more colloquial, but that often changes over time as they become more common. いらっしゃる itself was originally a reduced form of いらせらる, but it's now seen as the basic form of the word. いらして is still seen as a reduced form today, so speakers may perceive a difference between いらっしゃって and いらして, but this reduction is really extremely common, so I don't think it's especially marked.
Note that the initial い is sometimes elided as well, often following ～て:
ていらっしゃる 1695907 てらっしゃる 834652 ていらっしゃって 43510 てらっしゃって 22493 ていらっしゃった 132599 てらっしゃった 64947 ていらして 77646 てらして 59034 ていらした 158569 てらした 147049
This is similar to reducing ～ている to ～てる colloquially, although it's not quite as common. As you can see from the numbers above, eliding the い is especially frequent in combination with reducing いらっしゃっ to いらし.