Cullen is much more common than any of its variant spellings or similar surnames (except for Collins of course). Here are some of the different names/spellings found in the 1911 census of Ireland.
What is interesting about this? Some of the names that appear somewhat common in the U.S are rare or absent (Cullens and Cullins for example, also Cullum/Cullom is rare--maybe it's more of an English name?) Also names mentioned in the surname histories as being variants of Cullen are absent (Culloon, Culhoon).
If a number is not given, there were less than 10 people with that name in the census, usually only 1 or 2.
variant spellings and very rare surnames:
CULAN, CULLAN (66), CULLEEN, CULLEM, CULLENN, CULLAN, CULLENS (8 only), CULLEON, CULLION (96, mostly Donegal, some Tyrone), CULLLEN, CULLIM, CULLIN (82), CULLINS (10 only), CULLOM, CULLUM (15), CULLUMM, CULLON, CULHAM, CULHAN, CULLA (10), CULLAM, CULLANE (15), CULLANNE, CULLE, CULLEA, CULLEAN (10), CULLEANE, CULLEENY, CULLEHY, CULENEY, CULLER, CULLERY (10), CULLEW, CULLING, CULLINY, CULLIS, CULLNEY, CULLOHY, CULLOLY, CULLOO (17), CULLUANE, CULM, CULMEA, CULLINGNE, CULLMAN, CULLNEY, CULLOM, CULLON, CULLUM,
Then there are some names that are clearly different families, not just spelling variations:
CULHANE (611, mostly Limerick)
CULL (232, Down, Leitrim, Antrim and a few other counties)
CULLINEY (44, Mayo and Clare)
CULLINAN (863, Waterford to Roscommon)
CULLINANE (1240,Waterford to Roscommon)
Then there are the variant of McCullen:
McCULLEN (91), MacCULLEN (1), McCULL (17), McCULLION (12), McCULLAN (12), McCULLINS (11), McCULLIAN, McCULLIN, McCULLON, McCULLUM, McCULLAND, McCULLEM, MCULLIN
This list is only for names containing CUL, so names like COLLEN, COLLINS, KILLEN, QUILLEN, and McQUILLAN are not listed